Concurrent Direct Network Access for Virtual Machine - CiteSeer

12 Pages·2007·0.12 MB·English

ThispaperoriginallyappearedatHPCA2007
Concurrent Direct Network Access for Virtual Machine Monitors
PaulWillmann† JeffreyShafer† DavidCarr† AravindMenon‡
ScottRixner† AlanL.Cox† WillyZwaenepoel‡
†RiceUniversity ‡EPFL
Houston,TX Lausanne,Switzerland
{willmann,shafer,dcarr,rixner,alc}@rice.edu {aravind.menon,willy.zwaenepoel}@epfl.ch
Abstract machinesafelyandfairly. Inprinciple,general-purposeop-
eratingsystems,suchasUnixandWindows,offerthesame
Thispaperpresentshardwareandsoftwaremechanisms capability for multiple services to share the same physical
toenableconcurrentdirectnetworkaccess(CDNA)byop- machine. However, VMMsprovideadditional advantages.
erating systems running within a virtual machine monitor. Forexample,VMMsallowservicesimplementedindiffer-
In a conventional virtual machine monitor, each operating entorcustomizedenvironments,includingdifferentoperat-
system running within a virtual machine must access the ingsystems,tosharethesamephysicalmachine.
network through a software-virtualized network interface. Modern VMMs for commodity hardware, such as
Thesevirtualnetworkinterfacesaremultiplexedinsoftware VMWare[1,7]andXen[4],virtualizeprocessor,memory,
ontoaphysicalnetworkinterface,incurringsignificantper- and I/O devices in software. This enables these VMMs to
formanceoverheads. TheCDNAarchitectureimprovesnet- support a variety of hardware. In an attempt to decrease
workingefficiencyandperformancebydividingthetasksof thesoftwareoverheadofvirtualization,bothAMDandIntel
trafficmultiplexing,interruptdelivery,andmemoryprotec- areintroducinghardwaresupportforvirtualization[2,10].
tion between hardware and software in a novel way. The Specifically, their hardware support for processor virtual-
virtual machine monitor delivers interrupts and provides ization is currently available, and their hardware support
protectionbetweenvirtualmachines,whilethenetworkin- formemoryvirtualizationisimminent. Asthesehardware
terfaceperformsmultiplexingofthenetworkdata. Ineffect, mechanismsmature,theyshouldreducetheoverheadofvir-
the CDNA architecture provides the abstraction that each tualization,improvingtheefficiencyofVMMs.
virtualmachineisconnecteddirectlytoitsownnetworkin- Despite the renewed interest in system virtualization,
terface. ThroughtheuseofCDNA,manyofthebottlenecks there is still no clear solution to improve the efficiency of
imposed by software multiplexing can be eliminated with- I/O virtualization. To support networking, a VMM must
out sacrificing protection, producing substantial efficiency present each virtual machine with a virtual network inter-
improvements. face that is multiplexed in software onto a physical net-
work interface card (NIC). The overhead of this software-
basednetworkvirtualizationseverelylimitsnetworkperfor-
1 Introduction mance [12, 13, 19]. For example, a Linux kernel running
within a virtual machine on Xen is only able to achieve
about30%ofthenetworkthroughputthatthesamekernel
In many organizations, the economics of supporting
canachieverunningdirectlyonthephysicalmachine.
a growing number of Internet-based services has created
Thispaperproposesandevaluatesconcurrentdirectnet-
a demand for server consolidation. Consequently, there
work access (CDNA), a new I/O virtualization technique
has been a resurgence of interest in machine virtualiza-
combining both software and hardware components that
tion[1,2,4,7,9,10,11,19,22]. Avirtualmachinemoni-
significantlyreducestheoverheadofnetworkvirtualization
tor(VMM)enablesmultiplevirtualmachines,eachencap-
in VMMs. The CDNA network virtualization architecture
sulating one or more services, to share the same physical
provides virtual machines running on a VMM safe direct
ThisworkwassupportedinpartbytheTexasAdvancedTechnologyPro- access to the network interface. With CDNA, each virtual
gramunderGrantNo.003604-0078-2003,bytheNationalScienceFoun- machineisallocatedauniquecontextonthenetworkinter-
dationunderGrantNo.CCF-0546140,byagrantfromtheSwissNational
faceandcommunicatesdirectlywiththenetworkinterface
ScienceFoundation,andbygiftsfromAdvancedMicroDevices,Hewlett-
Packard,andXilinx. through that context. In this manner, the virtual machines
1
ThispaperoriginallyappearedatHPCA2007
that run on the VMM operate as if each has access to its Driver Domain(cid:13) Guest(cid:13)
owndedicatednetworkinterface. Back-End Drivers(cid:13) Domain 1(cid:13)
Front-End(cid:13)
Using CDNA, a single virtual machine running Linux Page(cid:13) Driver(cid:13) Guest(cid:13)
Ethernet(cid:13) Flipping(cid:13) Domain 2(cid:13)
cantransmitatarateof1867Mb/swith51%idletimeand Bridge(cid:13) Packet(cid:13) Front-End(cid:13)
Data(cid:13) Driver(cid:13)
receiveatarateof1874Mb/swith41%idletime. Incon-
trast, at 97% CPU utilization, Xen is only able to achieve
NIC Driver(cid:13) Virtual Interrupts(cid:13)
1602Mb/sfortransmitand1112Mb/sforreceive. Further-
more, with 24 virtual machines, CDNA can still transmit Driver(cid:13)
Control(cid:13)
and receive at a rate of over 1860 Mb/s, but with no idle Hypervisor(cid:13)
Interrupt Dispatch(cid:13)
t8i9m1e.MIbn/scaonndtrarestc,eXiveenatisaornaltyeoabfl5e5t8oMtrabn/ssmwiitthat2a4rvaitretuoafl Pt Dacaktae(cid:13)(cid:13) InterHruypptesr(cid:13)visor(cid:13) Control + Data(cid:13)
machines. NIC(cid:13) Control(cid:13) CPU / Memory / Disk / Other Devices(cid:13)
TheCDNAnetworkvirtualizationarchitectureachieves
thisdramaticincreaseinnetworkefficiencybydividingthe Figure1.Xenvirtualmachineenvironment.
tasksoftrafficmultiplexing,interruptdelivery,andmemory
protection among hardware and software in a novel way.
interruptsinthesystemandpassesthemontotheguestop-
Traffic multiplexing is performed directly on the network
erating systems, as appropriate. Finally, all I/O operations
interface, whereas interrupt delivery and memory protec-
gothroughXeninordertoensurefairandnon-overlapping
tionareperformedbytheVMMwithsupportfromthenet-
accesstoI/Odevicesbytheguests.
work interface. This division of tasks into hardware and
Figure1showstheorganizationoftheXenVMM.Xen
software components simplifies the overall software archi-
consistsoftwoelements: thehypervisorandthedriverdo-
tecture,minimizesthehardwareadditionstothenetworkin-
main.Thehypervisorprovidesanabstractionlayerbetween
terface,andaddressesthenetworkperformancebottlenecks
the virtual machines, called guest domains, and the actual
ofXen.
hardware,enablingeachguestoperatingsystemtoexecute
The remainder of this paper proceeds as follows. The
as if it were the only operating system on the machine.
nextsectiondiscussesnetworkingintheXenVMMinmore
However,theguestoperatingsystemscannotdirectlycom-
detail.Section3describeshowCDNAmanagestrafficmul-
municatewiththephysicalI/Odevices. Exclusiveaccessto
tiplexing,interruptdelivery,andmemoryprotectioninsoft-
thephysicaldevicesisgivenbythehypervisortothedriver
wareandhardwaretoprovideconcurrentaccesstotheNIC.
domain,aprivilegedvirtualmachine. Eachguestoperating
Section4thendescribesthecustomhardwareNICthatfa-
systemisthengivenavirtualI/Odevicethatiscontrolledby
cilitates concurrent direct network access on a single de-
aparavirtualizeddriver,calledafront-enddriver.Inorderto
vice. Section5presentstheexperimentalmethodologyand
accessaphysicaldevice,suchasthenetworkinterfacecard
results. Finally, Section 6 discusses related work and Sec-
(NIC), the guest’s front-end driver communicates with the
tion7concludesthepaper.
corresponding back-end driver in the driver domain. The
driver domain then multiplexes the data streams for each
2 NetworkinginXen guest onto the physical device. The driver domain runs
a modified version of Linux that uses native Linux device
driverstomanageI/Odevices.
2.1 HypervisorandDriverDomainOperation
Asthefigureshows,inordertoprovidenetworkaccess
to the guest domains, the driver domain includes a soft-
A VMM allows multiple guest operating systems, each ware Ethernet bridge that interconnects the physical NIC
runninginavirtualmachine,toshareasinglephysicalma- andallofthevirtualnetworkinterfaces. Whenapacketis
chinesafelyandfairly. Itprovidesisolationbetweenthese transmittedbyaguest,itisfirsttransferredtotheback-end
guest operating systems and manages their access to hard- driver in the driver domain using a page remapping oper-
wareresources. XenisanopensourceVMMthatsupports ation. Within the driver domain, the packet is then routed
paravirtualization,whichrequiresmodificationstotheguest through the Ethernet bridge to the physical device driver.
operatingsystem[4].Bymodifyingtheguestoperatingsys- The device driver enqueues the packet for transmission on
temstointeractwiththeVMM,thecomplexityoftheVMM thenetworkinterfaceasifitweregeneratednormallybythe
canbereducedandoverallsystemperformanceimproved. operatingsystemwithinthedriverdomain. Whenapacket
Xenperformsthreekeyfunctionsinordertoprovidevir- isreceived,thenetworkinterfacegeneratesaninterruptthat
tualmachineenvironments. First, Xenallocatesthephysi- iscapturedbythehypervisorandroutedtothenetworkin-
calresourcesofthemachinetotheguestoperatingsystems terface’s device driver in the driver domain as a virtual in-
andisolatesthemfromeachother. Second,Xenreceivesall terrupt. Thenetworkinterface’sdevicedrivertransfersthe
2
ThispaperoriginallyappearedatHPCA2007
packet to the Ethernet bridge, which routes the packet to System Transmit(Mb/s) Receive(Mb/s)
the appropriate back-end driver. The back-end driver then NativeLinux 5126 3629
transfersthepackettothefront-enddriverintheguestdo- XenGuest 1602 1112
mainusingapageremappingoperation. Oncethepacketis
transferred,theback-enddriverrequeststhatthehypervisor Table 1. Transmit and receive performance
send a virtual interrupt to the guest notifying it of the new fornativeLinux2.6.16.29andparavirtualized
packet. Upon receiving the virtual interrupt, the front-end Linux2.6.16.29asaguestOSwithinXen3.
driver delivers the packet to the guest operating system’s
networkstack, asifithadcomedirectlyfromthephysical
The network interface monitors these mailboxes for such
device.
writes from the host. When a mailbox update is detected,
the NIC reads the new producer value from the mailbox,
2.2 DeviceDriverOperation
performsaDMAreadofthedescriptorindicatedbythein-
dex,andthenisreadytousetheDMAdescriptor. Afterthe
The driver domain in Xen is able to use unmodified NICconsumesadescriptorfromaring,theNICupdatesits
Linuxdevicedriverstoaccessthenetworkinterface. Thus, consumerindex,transfersthisconsumerindextoalocation
all interactions between the device driver and the NIC are inhostmemoryviaDMA,andraisesaphysicalinterruptto
as they would be in an unvirtualized system. These inter- notifythehostthatstatehaschanged.
actionsincludeprogrammedI/O(PIO)operationsfromthe Inanunvirtualizedoperatingsystem, thenetworkinter-
driver to the NIC, direct memory access (DMA) transfers facetruststhatthedevicedrivergivesitvalidDMAdescrip-
bytheNICtoread orwritehostmemory, and physical in- tors. Similarly,thedevicedrivertruststhattheNICwilluse
terruptsfromtheNICtoinvokethedevicedriver. theDMAdescriptorscorrectly. Ifeitherentityviolatesthis
The device driver directs the NIC to send packets from trust,physicalmemorycanbecorrupted. Xenalsorequires
buffers in host memory and to place received packets into thistrustrelationshipbetweenthedevicedriverinthedriver
preallocated buffers in host memory. The NIC accesses domainandtheNIC.
thesebuffersusingDMAreadandwriteoperations. Inor-
derfortheNICtoknowwheretostoreorretrievedatafrom 2.3 Performance
thehost,thedevicedriverwithinthehostoperatingsystem
generates DMA descriptors for use by the NIC. These de- Despite the optimizations within the paravirtualized
scriptors indicate the buffer’s length and physical address drivers to support communication between the guest and
onthehost. ThedevicedrivernotifiestheNICviaPIOthat driver domains (such as using page remapping rather than
new descriptors are available, which causes the NIC to re- copyingtotransferpackets),Xenintroducessignificantpro-
trievethemviaDMAtransfers.OncetheNICreadsaDMA cessing and communication overheads into the network
descriptor,itcaneitherreadfromorwritetotheassociated transmit and receive paths. Table 1 shows the network-
buffer, depending on whether the descriptor is being used ing performance of both native Linux 2.6.16.29 and para-
bythedrivertotransmitorreceivepackets. virtualized Linux 2.6.16.29 as a guest operating system
Device drivers organize DMA descriptors in a series of within Xen 3 Unstable1 on a modern Opteron-based sys-
ringsthataremanagedusingaproducer/consumerprotocol. temwithsixIntelGigabitEthernetNICs. Inbothconfigu-
As they are updated, the producer and consumer pointers rations, checksum offloading, scatter/gather I/O, and TCP
wraparoundtheringstocreateacontinuouscircularbuffer. Segmentation Offloading (TSO) were enabled. Support
There are separate rings of DMA descriptors for transmit for TSO was recently added to the unstable development
andreceiveoperations. TransmitDMAdescriptorspointto branch of Xen and is not currently available in the Xen 3
host buffers that will be transmitted by the NIC, whereas release. As the table shows, a guest domain within Xen is
receive DMA descriptors point to host buffers that the OS onlyabletoachieveabout30%oftheperformanceofnative
wantstheNICtouseasitreceivespackets. Whenthehost Linux. This performance gap strongly motivates the need
driver wants to notify the NIC of the availability of a new fornetworkingperformanceimprovementswithinXen.
DMAdescriptor(andhenceanewpackettobetransmitted
oranewbuffertobepostedforpacketreception),thedriver
3 ConcurrentDirectNetworkAccess
first creates the new DMA descriptor in the next-available
slot in the driver’s descriptor ring and then increments the
With CDNA, the network interface and the hypervisor
producer index on the NIC to reflect that a new descriptor
collaboratetoprovidetheabstractionthateachguestoper-
is available. The driver updates the NIC’s producer index
ating system is connected directly to its own network in-
bywritingthevalueviaPIOintoaspecificlocation,called
amailbox,withinthedevice’sPCImemory-mappedregion. 1Changeset12053:874cc0ff214dfrom11/1/2006.
3
ThispaperoriginallyappearedatHPCA2007
terface. Thiseliminatesmanyoftheoverheadsofnetwork Guest(cid:13)
Domain 1(cid:13)
virtualization in Xen. Figure 2 shows the CDNA architec-
Guest(cid:13)
NIC Driver(cid:13)
ture. Thenetworkinterfacemustsupportmultiplecontexts Domain 2(cid:13)
Guest(cid:13)
in hardware. Each context acts as if it is an independent Virtual Interrupts(cid:13) NIC Driver(cid:13) Domain ...(cid:13)
physicalnetworkinterfaceandcanbecontrolledbyasepa- NIC Driver(cid:13)
ratedevicedriverinstance. Insteadofassigningownership Hypervisor(cid:13) DInisteprarutcpht(cid:13)(cid:13) Interrupts(cid:13) Packet(cid:13)
oftheentirenetworkinterfacetothedriverdomain,thehy- Data(cid:13) Driver(cid:13)
Control(cid:13) Control(cid:13)
pervisortreatseachcontextasifitwereaphysicalNICand
assigns ownership of contexts to guest operating systems. CPU / Memory / Disk(cid:13) CDNA NIC(cid:13)
Notice the absence of the driver domain from the figure:
eachguestcantransmitandreceivenetworktrafficusingits Figure2.CDNAarchitectureinXen.
ownprivatecontextwithoutanyinteractionwithotherguest
operatingsystemsorthedriverdomain. Thedriverdomain,
otherthanitsown. Whennecessary,thehypervisorcanalso
however, is still present to perform control functions and
revoke a context at any time by notifying the NIC, which
allowaccesstootherI/Odevices. Furthermore, thehyper-
will shut down all pending operations associated with the
visor is still involved in networking, as it must guarantee
indicatedcontext.
memoryprotectionanddelivervirtualinterruptstotheguest
To multiplex transmit network traffic, the NIC simply
operatingsystems.
services all of the hardware contexts fairly and interleaves
WithCDNA,thecommunicationoverheadsbetweenthe
the network traffic for each guest. When network pack-
guest and driver domains and the software multiplexing
ets are received by the NIC, it uses the Ethernet MAC ad-
overheadswithinthedriverdomainareeliminatedentirely.
dresstodemultiplexthetraffic,andtransferseachpacketto
However, the network interface now must multiplex the
theappropriateguestusingavailableDMAdescriptorsfrom
traffic across all of its active contexts, and the hypervisor
thatguest’scontext.
mustprovideprotectionacrossthecontexts. Thefollowing
sectionsdescribehowCDNAperformstrafficmultiplexing,
3.2 InterruptDelivery
interruptdelivery,andDMAmemoryprotection.
In addition to isolating the guest operating systems and
3.1 MultiplexingNetworkTraffic
multiplexing network traffic, the hardware contexts on the
NIC must also be able to interrupt their respective guests.
CDNA eliminates the software multiplexing overheads As the NIC carries out network requests on behalf of any
within the driver domain by multiplexing network traffic particular context, the CDNA NIC updates that context’s
ontheNIC.Thenetworkinterfacemustbeabletoidentify consumer pointers for the DMA descriptor rings, as de-
thesourceortargetguestoperatingsystemforallnetwork scribedinSection2.2. Normally,theNICwouldtheninter-
traffic. Thenetworkinterfaceaccomplishesthisbyprovid- rupttheguesttonotifyitthatthecontextstatehaschanged.
ingindependenthardwarecontextsandassociatingaunique However, inXenallphysicalinterruptsarehandledbythe
Ethernet MAC address with each context. The hypervisor hypervisor. Therefore, theNICcannotphysicallyinterrupt
assignsauniquehardwarecontextontheNICtoeachguest theguestoperatingsystemsdirectly. Evenifitwerepossi-
operatingsystem. Thedevicedriverwithintheguestoper- bletointerrupttheguestsdirectly,thatcouldcreateamuch
atingsystemtheninteractswithitscontextexactlyasifthe higherinterruptloadonthesystem, whichwoulddecrease
contextwereanindependentphysicalnetworkinterface.As theperformancebenefitsofCDNA.
describedinSection2.2,theseinteractionsconsistofcreat- Under CDNA, the NIC keeps track of which contexts
ing DMA descriptors and updating a mailbox on the NIC havebeenupdatedsincethelastphysicalinterrupt, encod-
viaPIO. ing this set of contexts in an interrupt bit vector. The NIC
Eachcontextonthenetworkinterfacethereforemustin- transfersaninterruptbitvectorintothehypervisor’smem-
clude a unique set of mailboxes. This isolates the activity ory space using DMA. The interrupt bit vectors are stored
ofeachguestoperatingsystem,sothattheNICcandistin- in a circular buffer using a producer/consumer protocol to
guishbetweenthedifferentguests. Thehypervisorassigns ensurethattheyareprocessedbythehostbeforebeingover-
acontexttoaguestsimplybymappingtheI/Olocationsfor written by the NIC. After an interrupt bit vector is trans-
thatcontext’smailboxesintotheguest’saddressspace. The ferred, the NIC raises a physical interrupt, which invokes
hypervisor also notifies the NIC that the context has been the hypervisor’s interrupt service routine. The hypervisor
allocated and is active. As the hypervisor only maps each then decodes all of the pending interrupt bit vectors and
context into a single guest’s address space, a guest cannot schedules virtual interrupts to each of the guest operating
accidentallyorintentionallyaccessanycontextontheNIC systemsthathavependingupdatesfromtheNIC.Whenthe
4
ThispaperoriginallyappearedatHPCA2007
guestoperatingsystemsarenextscheduledbythehypervi- vice.Thus,theuntrustedguestscouldreadorwritememory
sor,theCDNAnetworkinterfacedriverwithintheguestre- inanyotherdomainthroughtheNIC,unlessadditionalse-
ceivesthesevirtualinterruptsasiftheywereactualphysical curity features are added. To maintain isolation between
interruptsfromthehardware. Atthattime,thedriverexam- guests, the CDNA architecture validates and protects all
inestheupdatesfromtheNICanddetermineswhatfurther DMA descriptors and ensures that a guest maintains own-
action,suchasprocessingreceivedpackets,isrequired. ership of physical pages that are sources or targets of out-
standing DMA accesses. Although the hypervisor and the
3.3 DMAMemoryProtection networkinterfacesharetheresponsibilityforimplementing
theseprotectionmechanisms,themorecomplexaspectsare
implementedinthehypervisor.
Inthex86architecture,networkinterfacesandotherI/O
ThemostimportantprotectionprovidedbyCDNAisthat
devicesusephysicaladdresseswhenreadingorwritinghost
it does not allow guest domains to directly enqueue DMA
system memory. The device driver in the host operating
descriptors into the network interface descriptor rings. In-
system is responsible for doing virtual-to-physical address
stead,thedevicedriverineachguestmustcallintothehy-
translation for the device. The physical addresses are pro-
pervisortoperformtheenqueueoperation. Thisallowsthe
videdtothenetworkinterfacethroughreadandwriteDMA
hypervisor tovalidate thatthephysical addresses provided
descriptorsasdiscussedinSection2.2. Byexposingphys-
bytheguestare,infact,ownedbythatguestdomain. This
ical addresses to the network interface, the DMA engine
prevents aguestdomainfromarbitrarilytransmittingfrom
ontheNICcanbeco-optedintocompromisingsystemse-
orreceivingintoanotherguestdomain.Thehypervisorpre-
curity by a buggy or malicious driver. There are two key
ventsguestoperatingsystemsfromindependentlyenqueue-
I/Oprotectionviolationsthatarepossibleinthex86archi-
ing unauthorized DMA descriptors by establishing the hy-
tecture. First, the device driver could instruct the NIC to
pervisor’sexclusivewriteaccesstothehostmemoryregion
transmitpacketscontainingapayloadfromphysicalmem-
containingtheCDNAdescriptorringsduringdriverinitial-
orythatdoesnotcontainpacketsgeneratedbytheoperating
ization.
system,therebycreatingasecurityhole. Second,thedevice
drivercouldinstructtheNICtoreceivepacketsintophysi- AsdiscussedinSection2.2,conventionalI/Odevicesau-
calmemorythatwasnotdesignatedasanavailablereceive tonomously fetch and process DMA descriptors from host
buffer,possiblycorruptingmemorythatisinuse. memory at runtime. Though hypervisor-managed valida-
In the conventional Xen network architecture discussed tionandenqueuingofDMAdescriptorsensuresthatDMA
inSection2.2,Xentruststhedevicedriverinthedriverdo- operations are valid when they are enqueued, the physical
maintoonlyusethephysicaladdressesofnetworkbuffers memory could still be reallocated before it is accessed by
in the driver domain’s address space when passing DMA thenetworkinterface. Therearetwowaysinwhichsucha
descriptors to the network interface. This ensures that all protectionviolationcouldbeexploitedbyabuggyormali-
network traffic will be transferred to/from network buffers ciousdevicedriver.First,theguestcouldreturnthememory
withinthedriverdomain. Sinceguestdomainsdonotinter- tothehypervisortobereallocatedshortlyafterenqueueing
act with the NIC, they cannot initiate DMA operations, so the DMA descriptor. Second, the guest could attempt to
theyarepreventedfromcausingeitheroftheI/Oprotection reuse an old DMA descriptor in the descriptor ring that is
violationsinthex86architecture. nolongervalid.
Though the Xen I/O architecture guarantees that un- When memory is freed by a guest operating system, it
trustedguestdomainscannotinducememoryprotectionvi- becomes available for reallocation to another guest by the
olations, any domain that is granted access to an I/O de- hypervisor. Hence, ownership of the underlying physical
vice by the hypervisor can potentially direct the device to memory can change dynamically at runtime. However, it
performDMAoperationsthataccessmemorybelongingto is critical to prevent any possible reallocation of physical
other guests, or even the hypervisor. The Xen architecture memoryduringaDMAoperation. CDNAachievesthisby
doesnotfundamentallysolvethissecuritydefectbutinstead delaying the reallocation of physical memory that is being
limits the scope of the problem to a single, trusted driver used in a DMA transaction until after that pending DMA
domain[9]. Therefore,asthedriverdomainistrusted,itis hascompleted. WhenthehypervisorenqueuesaDMAde-
unlikelytointentionallyviolateI/Omemoryprotection,but scriptor, it first establishes that the requesting guest owns
a buggy driver within the driver domain could do so unin- the physical memory associated with the requested DMA.
tentionally. Thehypervisorthenincrementsthereferencecountforeach
This solution is insufficient for the CDNA architecture. physicalpageassociatedwiththerequestedDMA.Thisper-
In a CDNA system, device drivers in the guest domains page reference counting system already exists within the
have direct access to the network interface and are able to Xenhypervisor;solongasthereferencecountisnon-zero,
pass DMA descriptors with physical addresses to the de- aphysicalpagecannotbereallocated. Later,thehypervisor
5
ThispaperoriginallyappearedatHPCA2007
then observes whichDMAoperations have completed and mat.Fortunately,thereareonlythreefieldsofinterestinany
decrementstheassociatedreferencecounts. Forefficiency, DMAdescriptor: anaddress,alength,andadditionalflags.
thereferencecountsareonlydecrementedwhenadditional Thiscommonalityshouldmakeitpossibletogeneralizethe
DMAdescriptorsareenqueued,butthereisnoreasonwhy mechanismswithinthehypervisorbyhavingtheNICnotify
theycouldnotbedecrementedmoreaggressively,ifneces- thehypervisorofitspreferredformat. TheNICwouldonly
sary. need to specify the size of the descriptor and the location
AfterenqueuingDMAdescriptors,thedevicedriverno- oftheaddress,length,andflags. Thehypervisorwouldnot
tifies the NIC by writing a producer index into a mailbox needtointerprettheflags,sotheycouldjustbecopiedinto
location within that guest’s context on the NIC. This pro- the appropriate location. A generic NIC would also need
ducer index indicates the location of the last of the newly to support the use of sequence numbers within each DMA
created DMA descriptors. The NIC then assumes that all descriptor. Again, the NIC could notify the hypervisor of
DMA descriptors up to the location indicated by the pro- the size and location of the sequence number field within
ducerindexarevalid. Ifthedevicedriverintheguestincre- thedescriptors.
ments theproducer index pastthelastvaliddescriptor, the CDNA’s DMA memory protection is specific to Xen
NICwillattempttouseastaleDMAdescriptorthatisinthe onlyinsofarasXenpermitsguestoperatingsystemstouse
descriptor ring. Since that descriptor was previously used physicalmemoryaddresses. Consequently,thecurrentim-
inaDMAoperation,thehypervisormayhavedecremented plementationmustvalidatetheownershipofthosephysical
thereferencecountontheassociatedphysicalmemoryand addressesforeveryrequestedDMAoperation. ForVMMs
reallocatedthephysicalmemory. that only permit the guest to use virtual addresses, the hy-
TopreventsuchstaleDMAdescriptorsfrombeingused, pervisorcouldjustaseasilytranslatethosevirtualaddresses
the hypervisor writes a strictly increasing sequence num- and ensure physical contiguity. The current CDNA imple-
ber into each DMA descriptor. The NIC then checks the mentation does not rely on physical addresses in the guest
sequencenumberbeforeusinganyDMAdescriptor. Ifthe at all; rather, a small library translates the driver’s virtual
descriptorisvalid,thesequencenumberswillbecontinuous addressestophysicaladdresseswithintheguest’sdriverbe-
modulothesizeofthemaximumsequencenumber. Ifthey foremakingahypercallrequesttoenqueueaDMAdescrip-
arenot, theNICwillrefusetousethedescriptorsandwill tor.ForVMMsthatusevirtualaddresses,thislibrarywould
reportaguest-specificprotectionfaulterrortothehypervi- donothing.
sor. Because each DMA descriptor in the ring buffer gets
a new, increasing sequence number, a stale descriptor will 4 CDNANICImplementation
have a sequence number exactly equal to the correct value
minus the number of descriptor slots in the buffer. Mak-
To evaluate the CDNA concept in a real system,
ing the maximum sequence number at least twice as large
RiceNIC,aprogrammableandreconfigurableFPGA-based
asthenumberofDMAdescriptorsinaringbufferprevents
Gigabit Ethernet network interface [17], was modified to
aliasingandensuresthatanystalesequencenumberwillbe
providevirtualizationsupport. RiceNICcontainsaVirtex-
detected.
II Pro FPGA with two embedded 300MHz PowerPC pro-
cessors, hundreds of megabytes of on-board SRAM and
3.4 Discussion
DRAM memories, a Gigabit Ethernet PHY, and a 64-
bit/66MHzPCIinterface[3]. Customhardwareassistunits
The CDNA interrupt delivery mechanism is neither de- for accelerated DMA transfers and MAC packet handling
vice nor Xen specific. This mechanism only requires the are provided on the FPGA. The RiceNIC architecture is
device to transfer an interrupt bit vector to the hypervisor similar to the architecture of a conventional network in-
viaDMApriortoraisingaphysicalinterrupt. Thisisarela- terface. With basic firmware and the appropriate Linux or
tivelysimplemechanismfromtheperspectiveofthedevice FreeBSDdevicedriver,itactsasastandardGigabitEther-
andisthereforegeneralizabletoavarietyofvirtualizedI/O netnetworkinterfacethatiscapableoffullysaturatingthe
devices. Furthermore, itdoesnotrelyonanyXen-specific Ethernet link while only using one of the two embedded
features. processors.
ThehandlingoftheDMAdescriptorswithinthehyper- To support CDNA, both the hardware and firmware of
visor is linked to a particular network interface only be- the RiceNIC were modified to provide multiple protected
cause the format of the DMA descriptors and their rings contexts and to multiplex network traffic. The network
is likely to be different for each device. As the hypervisor interface was also modified to interact with the hypervi-
mustvalidatethatthehostaddressesreferredtoineachde- sor through a dedicated context to allow privileged man-
scriptorbelongtotheguestoperatingsystemthatprovided agement operations. The modified hardware and firmware
them, the hypervisor must be aware of the descriptor for- components work together to implement the CDNA inter-
6
ThispaperoriginallyappearedatHPCA2007
faces. normal operation of the network interface—unvirtualized
To support CDNA, the most significant addition to the devicedriverswoulduseasinglecontext’smailboxestoin-
networkinterfaceisthespecializeduseofthe2MBSRAM teract with the base firmware. Furthermore, the computa-
ontheNIC.ThisSRAMisaccessibleviaPIOfromthehost. tionandstoragerequirementsofCDNAareminimal. Only
ForCDNA,128KBoftheSRAMisdividedinto32parti- one of the RiceNIC’s two embedded processors is needed
tionsof4KBeach. Eachofthesepartitionsisaninterface tosaturatethenetwork,andonly12MBofmemoryonthe
toaseparatehardwarecontextontheNIC.OnlytheSRAM NICisneededtosupport32contexts. Therefore, withmi-
canbememorymappedintothehost’saddressspace,sono normodifications,commoditynetworkinterfacescouldeas-
othermemorylocationsontheNICareaccessibleviaPIO. ilyprovidesufficientcomputationandstorageresourcesto
Asacontext’smemorypartitionisthesamesizeasapage supportCDNA.
onthehostsystemandbecausetheregionispage-aligned,
thehypervisor can triviallymapeach context intoadiffer- 5 Evaluation
entguestdomain’saddressspace. Thedevicedriversinthe
guestdomainsmayusethese4KBpartitionsasgeneralpur- 5.1 ExperimentalSetup
pose shared memory between the corresponding guest op-
eratingsystemandthenetworkinterface. The performance of Xen and CDNA network virtual-
Within each context’s partition, the lowest 24 memory ization was evaluated on an AMD Opteron-based system
locations are mailboxes that can be used to communicate running Xen 3Unstable2. This systemused a Tyan S2882
from the driver to the NIC. When any mailbox is written motherboardwithasingleOpteron250processorand4GB
by PIO, a global mailbox event is automatically generated ofDDR400SDRAM.Xen3Unstablewasusedbecauseit
by the FPGA hardware. The NIC firmware can then pro- provides the latest support for high-performance network-
cesstheeventandefficientlydeterminewhichmailboxand ing, including TCP segmentation offloading, and the most
correspondingcontexthasbeenwrittenbydecodingatwo- recentversionofXenoprof[13]forprofilingtheentiresys-
levelhierarchyofbitvectors. Allofthebitvectorsaregen- tem.
erated automatically by the hardware and stored in a data In all experiments, the driver domain was configured
scratchpadforhighspeedaccessbytheprocessor. Thefirst with256MBofmemoryandeachof24guestdomainswere
bit vector in the hierarchy determines which of the 32 po- configuredwith128MBofmemory.Eachguestdomainran
tentialcontextshaveupdatedmailboxeventstoprocess,and a stripped-down Linux 2.6.16.29 kernel with minimal ser-
the second vector in the hierarchy determines which mail- vicesformemoryefficiencyandperformance. Forthebase
box(es)inaparticularcontexthavebeenupdated. Oncethe Xenexperiments,asingledual-portIntelPro/1000MTNIC
specific mailbox has been identified, that off-chip SRAM was used in the system. In the CDNA experiments, two
locationcanbereadbythefirmwareandthemailboxinfor- RiceNICs configured to support CDNA were used in the
mationprocessed. system. LinuxTCPparametersandNICcoalescingoptions
The mailbox event and associated hierarchy of bit vec- weretunedinthedriverdomainandguestdomainsforopti-
tors are managed by a small hardware core that snoops malperformance.Forallexperiments,checksumoffloading
dataontheSRAMbusanddispatchesnotificationmessages andscatter/gatherI/Owereenabled. TCPsegmentationoff-
whenamailboxisupdated. Asmallstatemachinedecodes loading was enabled for experiments using the Intel NICs,
thesemessagesandincrementallyupdatesthedatascratch- but disabled for those using the RiceNICs due to lack of
pad with the modified bit vectors. This state machine also support. TheXensystemwassetuptocommunicatewitha
handles event-clear messages from the processor that can similarOpteronsystemthatwasrunninganativeLinuxker-
clearmultipleeventsfromasinglecontextatonce. nel. This system was tuned so that it could easily saturate
two NICs both transmitting and receiving so that it would
Each context requires 128 KB of storage on the NIC
neverbethebottleneckinanyofthetests.
for metadata, such as the rings of transmit- and receive-
To validate the performance of the CDNA approach,
DMA descriptors provided by the host operating systems.
multiplesimultaneousconnectionsacrossmultipleNICsto
Furthermore, eachcontext uses128KBofmemoryonthe
multiple guests domains were needed. A multithreaded,
NICforbufferingtransmitpacketdataand128 KBforre-
event-driven,lightweightnetworkbenchmarkprogramwas
ceivepacketdata. However,theNIC’stransmitandreceive
developedtodistributetrafficacrossaconfigurablenumber
packetbuffersareeachmanagedglobally,andhencepacket
ofconnections.Thebenchmarkprogrambalancestheband-
bufferingissharedacrossallcontexts.
width across all connections to ensure fairness and uses a
The modifications to the RiceNIC to support CDNA
singlebufferperthreadtosendandreceivedatatominimize
were minimal. The major hardware change was the addi-
thememoryfootprintandimprovecacheperformance.
tionalmailboxstorageandhandlinglogic. Thiscouldeas-
ilybeaddedtoanexistingNICwithoutinterferingwiththe 2Changeset12053:874cc0ff214dfrom11/1/2006.
7
ThispaperoriginallyappearedatHPCA2007
DomainExecutionProfile Interrupts/s
System NIC Mb/s DriverDomain GuestOS Driver Guest
Hyp Idle
OS User OS User Domain OS
Xen Intel 1602 19.8% 35.7% 0.8% 39.7% 1.0% 3.0% 7,438 7,853
Xen RiceNIC 1674 13.7% 41.5% 0.5% 39.5% 1.0% 3.8% 8,839 5,661
CDNA RiceNIC 1867 10.2% 0.3% 0.2% 37.8% 0.7% 50.8% 0 13,659
Table2.Transmitperformanceforasingleguestwith2NICsusingXenandCDNA.
DomainExecutionProfile Interrupts/s
System NIC Mb/s DriverDomain GuestOS Driver Guest
Hyp Idle
OS User OS User Domain OS
Xen Intel 1112 25.7% 36.8% 0.5% 31.0% 1.0% 5.0% 11,138 5,193
Xen RiceNIC 1075 30.6% 39.4% 0.6% 28.8% 0.6% 0% 10,946 5,163
CDNA RiceNIC 1874 9.9% 0.3% 0.2% 48.0% 0.7% 40.9% 0 7,402
Table3.Receiveperformanceforasingleguestwith2NICsusingXenandCDNA.
5.2 SingleGuestPerformance Table 2 shows that using all of the available processing
resources,Xen’ssoftwarevirtualizationisnotabletotrans-
mitatlinerateover twonetwork interfaceswitheitherthe
Tables 2 and 3 show the transmit and receive perfor-
Intel hardware or the RiceNIC hardware. However, only
mance of a single guest operating system over two physi-
41%oftheprocessorisusedbytheguestoperatingsystem.
calnetworkinterfacesusingXenandCDNA.Thefirsttwo
TheremainingresourcesareconsumedbyXenoverheads—
rows of each table show the performance of the Xen I/O
usingtheIntelhardware, approximately20%inthehyper-
virtualizationarchitectureusingboththeIntelandRiceNIC
visor and 37% in the driver domain performing software
network interfaces. The third row of each table shows the
multiplexingandothertasks.
performanceoftheCDNAI/Ovirtualizationarchitecture.
As the table shows, CDNA is able to saturate two net-
The Intel network interface can only be used with Xen
work interfaces, whereas traditional Xen networking can-
through the use of software virtualization. However, the
not. Additionally, CDNA performs far more efficiently,
RiceNICcanbeusedwithbothCDNAandsoftwarevirtu-
with 51% processor idle time. The increase in idle time
alization. TousetheRiceNICinterfacewithsoftwarevirtu-
is primarily the result of two factors. First, nearly all of
alization, a context was assigned to the driver domain and
the time spent in the driver domain is eliminated. The re-
no contexts were assigned to the guest operating system.
mainingtimespentinthedriverdomainisunrelatedtonet-
Therefore,allnetworktrafficfromtheguestoperatingsys-
working tasks. Second, the time spent in the hypervisor is
temisroutedviathedriverdomainasitnormallywouldbe,
decreased. WithXen,thehypervisorspendsthebulkofits
throughtheuseofsoftwarevirtualization. Withinthedriver
time managing the interactions between the front-end and
domain, all of the mechanisms within the CDNA NIC are
back-end virtual network interface drivers. CDNA elimi-
used identically to the way they would be used by a guest
nates these communication overheads with the driver do-
operatingsystemwhenconfiguredtouseconcurrentdirect
main, sothehypervisorinsteadspendsthebulkofitstime
networkaccess. Asthetablesshow,theIntelnetworkinter-
managingDMAmemoryprotection.
face performs similarly to the RiceNIC network interface.
Therefore, the benefits achieved with CDNA are the result Table3showsthereceiveperformanceofthesamecon-
of the CDNA I/O virtualization architecture, not the result figurations. Receiving network traffic requires more pro-
ofdifferencesinnetworkinterfaceperformance. cessorresources,soXenonlyachieves1112Mb/swiththe
Intelnetworkinterface,andslightlylowerwiththeRiceNIC
NotethatinXentheinterruptratefortheguestisnotnec-
interface. Again, Xen overheads consume the bulk of the
essarilythesameasitisforthedriver. Thisisbecausethe
time, as the guest operating system only consumes about
back-enddriverwithinthedriverdomainattemptstointer-
32% of the processor resources when using the Intel hard-
rupttheguestoperatingsystemwhenever itgenerates new
ware.
work for the front-end driver. This can happen at a higher
orlowerratethantheactualinterruptrategeneratedbythe As the table shows, not only is CDNA able to saturate
networkinterfacedependingonavarietyoffactors,includ- the two network interfaces, it does so with 41% idle time.
ingthenumberofpacketsthattraversetheEthernetbridge Again, nearly all of the time spent in the driver domain is
eachtimethedriverdomainisscheduledbythehypervisor. eliminated. As with the transmit case, the CDNA archi-
8
ThispaperoriginallyappearedatHPCA2007
DomainExecutionProfile Interrupts/s
System DMAProtection Mb/s DriverDomain GuestOS Driver Guest
Hyp Idle
OS User OS User Domain OS
CDNA(Transmit) Enabled 1867 10.2% 0.3% 0.2% 37.8% 0.7% 50.8% 0 13,659
CDNA(Transmit) Disabled 1867 1.9% 0.2% 0.2% 37.0% 0.3% 60.4% 0 13,680
CDNA(Receive) Enabled 1874 9.9% 0.3% 0.2% 48.0% 0.7% 40.9% 0 7,402
CDNA(Receive) Disabled 1874 1.9% 0.2% 0.2% 47.2% 0.3% 50.2% 0 7,243
Table4.CDNA2-NICtransmitandreceiveperformancewithandwithoutDMAmemoryprotection.
tecturepermitsthehypervisortospenditstimeperforming spentinthehypervisorperformingprotectionoperations.
DMAmemoryprotectionratherthanmanaginghigher-cost Even as systems begin to provide IOMMU support for
interdomain communications as is required using software techniques such as CDNA, older systems will continue
virtualization. to lack such features. In order to generalize the design
In summary, the CDNA I/O virtualization architecture of CDNA for systems with and without an appropriate
provides significant performance improvements over Xen IOMMU, wrapper functions could be used around the hy-
for both transmit and receive. On the transmit side, percalls within the guest device drivers. The hypervisor
CDNArequireshalftheprocessorresourcestodeliverabout must notify the guest whether or not there is an IOMMU.
200 Mb/s higher throughput. On the receive side, CDNA When no IOMMU is present, the wrappers would simply
requires 60% of the processor resources to deliver about callthehypervisor,asdescribedhere. WhenanIOMMUis
750Mb/shigherthroughput. present,thewrapperwouldinsteadcreateDMAdescriptors
withouthypervisorinterventionandonlyinvokethehyper-
5.3 MemoryProtection visor to set up the IOMMU. Such wrappers already exist
inmodernoperatingsystemstodealwithsuchIOMMUis-
sues.
The software-based protection mechanisms in CDNA
can potentially be replaced by a hardware IOMMU. For
example, AMD has proposed an IOMMU architecture for 5.4 Scalability
virtualizationthatrestrictsthephysicalmemorythatcanbe
accessedbyeachdevice[2]. AMD’sproposedarchitecture Figures3and4showtheaggregatetransmitandreceive
providesmemoryprotectionaslongaseachdeviceisonly throughput, respectively, of Xen and CDNA with two net-
accessedbyasingledomain. ForCDNA,suchanIOMMU work interfaces as the number of guest operating systems
would have to be extended to work on a per-context basis, varies. The percentage of CPU idle time is also plotted
rather than a per-device basis. This would also require a above each data point. CDNA outperforms Xen for both
mechanism to indicate a context for each DMA transfer. transmit and receive both for a single guest, as previously
Since CDNA only distinguishes between guest operating showninTables2and3,andasthenumberofguestoper-
systemsandnottrafficflows,therearealimitednumberof atingsystemsisincreased.
contexts, which may make a generic system-level context- Asthefiguresshow,theperformanceofbothCDNAand
awareIOMMUpractical. softwarevirtualizationdegradesasthenumberofguestsin-
Table4showstheperformanceoftheCDNAI/Ovirtu- creases. For Xen, this results in declining bandwidth, but
alizationarchitecturebothwithandwithoutDMAmemory themarginalreductioninbandwidthdecreaseswitheachin-
protection. (The performance of CDNA with DMA mem- creaseinthenumberofguests. ForCDNA,whiletheband-
ory protection enabled was replicated from Tables 2 and 3 widthremainsconstant,theidletimedecreasestozero. De-
forcomparisonpurposes.)BydisablingDMAmemorypro- spitethefactthatthereisnoidletimefor8ormoreguests,
tection,theperformanceofthemodifiedCDNAsystemes- CDNA is still able to maintain constant bandwidth. This
tablishes an upper bound on achievable performance in a is consistent with the leveling of the bandwidth achieved
systemwithanappropriateIOMMU.However,therewould by software virtualization. Therefore, it is likely that with
be additional hypervisor overhead to manage the IOMMU moreCDNANICs,thethroughputcurvewouldhaveasim-
thatisnotaccounted forbythisexperiment. SinceCDNA ilarshapetothatofsoftwarevirtualization,butwithamuch
can already saturate two network interfaces for both trans- higherpeakthroughputwhenusing1–4guests.
mitandreceivetraffic,theeffectofremovingDMAprotec- TheseresultsclearlyshowthatnotonlydoesCDNAde-
tion is to increase the idle time by about 9%. As the table liverbetternetworkperformanceforasingleguestoperat-
shows,thisincreaseinidletimeisthedirectresultofreduc- ing system within Xen, but it also maintains significantly
ing the number of hypercalls from the guests and the time higherbandwidthasthenumberofguestoperatingsystems
9
ThispaperoriginallyappearedatHPCA2007
2000 50.8%25.4% 5.9% 0% 0% 0% 0% 0% 2000 40.9%29.1% 12.6% 0% 0% 0% 0% 0%
1800 1800
Mbps)1600 3.0% 0% Mbps)1600
Xen Transmit Throughput (111802400000000 0% 0% 0% 0% CXeDnN /A0 I% n/ tReliceNIC0% Xen Receive Throughput (111802400000000 5.0% 0% 0% 0% CXeDnN /A I n/ tReliceNIC
600 600 0% 0% 0% 0%
400 400
1 2 4 8 12 16 20 24 1 2 4 8 12 16 20 24
Xen Guests Xen Guests
Figure3.TransmitthroughputforXenand Figure 4. Receive throughput for Xen and
CDNA(withCDNAidletime). CDNA(withCDNAidletime).
is increased. With 24 guest operating systems, CDNA’s amongguestoperatingsystemsandtoenablethehypervisor
transmitbandwidthisafactorof2.1higherthanXen’sand tooccupyanewprivilegeleveldistinctfromthosenormally
CDNA’s receive bandwidth is a factor of 3.3 higher than used by the operating system. These improvements will
Xen’s. reduce the duration and frequency of calls into the hyper-
visor, which should decrease the performance overhead of
6 RelatedWork virtualization. However, none of the proposed innovations
directly address the network performance issues discussed
inthispaper,suchastheinherentoverheadinmultiplexing
Previousstudieshavealsofoundthatnetworkvirtualiza-
and copying/remapping data between the guest and driver
tion implemented entirely in software has high overhead.
domains. While the context switches between the two do-
In 2001, Sugerman, et al. showed that in VMware, it
mains may be reduced in number or accelerated, the over-
could take up to six times the processor resources to satu-
head ofcommunication and multiplexing within thedriver
rate a 100 Mb/s network than in native Linux [19]. Sim-
domain will remain. Therefore, concurrent direct network
ilarly, in 2005, Menon, et al. showed that in Xen, net-
access will continue to be an important element of VMMs
workthroughputdegradesbyuptoafactorof5overnative
fornetworkingworkloads.
Linuxforprocessor-boundnetworkingworkloadsusingGi-
gabit Ethernet links [13]. Section 2.3 shows that the I/O VMMs that utilize full virtualization, such as VMware
performance of Xen has improved, but there is still signif- ESX Server [7], support fullbinary compatibility withun-
icant network virtualization overhead. Menon, et al. have modifiedguestoperatingsystems. ThisimpactstheI/Ovir-
also shown that it is possible to improve transmit perfor- tualization architecture of such systems, as the guest op-
mance with software-only mechanisms (mainly by lever- erating system must be able to use its unmodified native
aging TSO) [12]. However, there are no known software devicedrivertoaccessthevirtualnetworkinterface. How-
mechanismstosubstantivelyimprovereceiveperformance. ever, VMware also allows the use of paravirtualized net-
Motivatedbytheseperformanceissues,RajandSchwan workdrivers(i.e., vmxnet), whichenables theuseoftech-
presentedanEthernetnetworkinterfacetargetedatVMMs niquessuchasCDNA.
that performs traffic multiplexing and interrupt deliv- The CDNA architecture is similar to that of user-level
ery[16].Whiletheirproposedarchitecturebearssomesim- networkingarchitecturesthatallowprocessestobypassthe
ilarity to CDNA, they did not present any mechanism for operating system and access the NIC directly [5, 6, 8, 14,
DMAmemoryprotection. 15, 18, 20, 21]. Like CDNA, these architectures require
As a result of the growing popularity of VMMs for DMAmemoryprotection,aninterruptdeliverymechanism,
commodity hardware, both AMD and Intel are introduc- andnetworktrafficmultiplexing. Bothuser-levelnetwork-
ing virtualization support to their microprocessors [2, 10]. ingarchitecturesandCDNAhandletrafficmultiplexingon
Thisvirtualizationsupportshouldimprovetheperformance the network interface. The only difference is that user-
of VMMs by providing mechanisms to simplify isolation levelNICshandleflowsonaper-applicationbasis,whereas
10