Bug 1308395

What is the change between kernel 3.10.0-327.el7.x86_64 between with 3.10.0-229.el7.x86_64 about multi-queue virtio-net suport? 
How many queues at most does 327 kernel supports for a multi-queue tap device?

I know nothing on that topic. Michael, do you know the answer to this?

according to the change log:
        It is now safe to increase the maximum number of queues.
        Preceding patch: 'net: allow large number of rx queues'
        made sure this won't cause failures due to high order memory
        allocations. Increase it to 256: this is the max number of vCPUs
        KVM supports.

Hi Micheal,

Thank you for your answer!
And I have tried with 256~249, found that once the queues larger than 250, the guest can not start, if it need particular hardware? Please help to confirm.

# rpm -q libvirt
libvirt-1.3.2-1.el7.x86_64
# uname -a
Linux work 3.10.0-342.el7.x86_64
# rpm -q qemu-kvm-rhev
qemu-kvm-rhev-2.3.0-31.el7_2.5.x86_64


# virsh dumpxml rhel7.2 | grep /interface -B4
    ...
      <driver name='vhost' queues='250'/>
      <address type='pci' domain='0x0000' bus='0x01' slot='0x01' function='0x0'/>
    </interface>
# virsh start rhel7.2
Domain rhel7.2 started
# ps -aux | grep mq=on | grep -v grep
qemu     14273  105  0.9 1701484 70760 ?       Sl   19:37   0:03 /usr/libexec/qemu-kvm -name rhel7.2 -S -machine pc-i440fx-rhel7.2.0,accel=kvm,usb=off,vmport=off -cpu Penryn,+osxsave,+xsave,+pdcm,+xtpr,+tm2,+est,+vmx,+ds_cpl,+monitor,+dtes64,+pbe,+tm,+ht,+ss,+acpi,+ds,+vme -m 1024 -realtime mlock=off -smp 2,sockets=2,cores=1,threads=1 -uuid c8529c02-7851-4f4a-b480-f777f72e3133 -no-user-config -nodefaults -chardev socket,id=charmonitor,path=/var/lib/libvirt/qemu/domain-rhel7.2/monitor.sock,server,nowait -mon chardev=charmonitor,id=monitor,mode=control -rtc base=utc,driftfix=slew -global kvm-pit.lost_tick_policy=discard -no-hpet -no-shutdown -global PIIX4_PM.disable_s3=1 -global PIIX4_PM.disable_s4=1 -boot strict=on -device pci-bridge,chassis_nr=1,id=pci.1,bus=pci.0,addr=0x1f -device ich9-usb-ehci1,id=usb,bus=pci.0,addr=0x6.0x7 -device ich9-usb-uhci1,masterbus=usb.0,firstport=0,bus=pci.0,multifunction=on,addr=0x6 -device ich9-usb-uhci2,masterbus=usb.0,firstport=2,bus=pci.0,addr=0x6.0x1 -device ich9-usb-uhci3,masterbus=usb.0,firstport=4,bus=pci.0,addr=0x6.0x2 -device virtio-serial-pci,id=virtio-serial0,bus=pci.0,addr=0x5 -drive file=/var/lib/libvirt/images/rhel7.2.qcow2,format=qcow2,if=none,id=drive-virtio-disk0 -device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x7,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1 -netdev tap,fds=25:27:28:29:30:31:32:33:34:35:36:37:38:39:40:41:42:43:44:45:46:47:48:49:50:51:52:53:54:55:56:57:58:59:60:61:62:63:64:65:66:67:68:69:70:71:72:73:74:75:76:77:78:79:80:81:82:83:84:85:86:87:88:89:90:91:92:93:94:95:96:97:98:99:100:101:102:103:104:105:106:107:108:109:110:111:112:113:114:115:116:117:118:119:120:121:122:123:124:125:126:127:128:129:130:131:132:133:134:135:136:137:138:139:140:141:142:143:144:145:146:147:148:149:150:151:152:153:154:155:156:157:158:159:160:161:162:163:164:165:167:168:169:170:171:172:173:174:175:176:177:178:179:180:181:182:183:184:185:186:187:188:189:190:191:192:193:194:195:196:197:198:199:200:201:202:203:204:205:206:207:208:209:210:211:212:213:214:215:216:217:218:219:220:221:222:223:224:225:226:227:228:229:230:231:232:233:234:235:236:237:238:239:240:241:242:243:244:245:246:247:248:249:250:251:252:253:254:255:256:257:258:259:260:261:262:263:264:265:266:267:268:269:270:271:272:273:274:275:276,id=hostnet0,vhost=on,vhostfds=166:277:278:279:280:281:282:283:284:285:286:287:288:289:290:291:292:293:294:295:296:297:298:299:300:301:302:303:304:305:306:307:308:309:310:311:312:313:314:315:316:317:318:319:320:321:322:323:324:325:326:327:328:329:330:331:332:333:334:335:336:337:338:339:340:341:342:343:344:345:346:347:348:349:350:351:352:353:354:355:356:357:358:359:360:361:362:363:364:365:366:367:368:369:370:371:372:373:374:375:376:377:378:379:380:381:382:383:384:385:386:387:388:389:390:391:392:393:394:395:396:397:398:399:400:401:402:403:404:405:406:407:408:409:410:411:412:413:414:415:416:417:418:419:420:421:422:423:424:425:426:427:428:429:430:431:432:433:434:435:436:437:438:439:440:441:442:443:444:445:446:447:448:449:450:451:452:453:454:455:456:457:458:459:460:461:462:463:464:465:466:467:468:469:470:471:472:473:474:475:476:477:478:479:480:481:482:483:484:485:486:487:488:489:490:491:492:493:494:495:496:497:498:499:500:501:502:503:504:505:506:507:508:509:510:511:512:513:514:515:516:517:518:519:520:521:522:523:524:525 -device virtio-net-pci,mq=on,vectors=502,netdev=hostnet0,id=net0,mac=52:54:00:e2:c5:73,bus=pci.1,addr=0x1 -chardev pty,id=charserial0 -device isa-serial,chardev=charserial0,id=serial0 -chardev socket,id=charchannel0,path=/var/lib/libvirt/qemu/channel/target/domain-rhel7.2/org.qemu.guest_agent.0,server,nowait -device virtserialport,bus=virtio-serial0.0,nr=1,chardev=charchannel0,id=channel0,name=org.qemu.guest_agent.0 -chardev spicevmc,id=charchannel1,name=vdagent -device virtserialport,bus=virtio-serial0.0,nr=2,chardev=charchannel1,id=channel1,name=com.redhat.spice.0 -device usb-tablet,id=input0 -spice port=5900,addr=127.0.0.1,disable-ticketing,image-compression=off,seamless-migration=on -device qxl-vga,id=video0,ram_size=67108864,vram_size=67108864,vgamem_mb=16,bus=pci.0,addr=0x2 -device intel-hda,id=sound0,bus=pci.0,addr=0x4 -device hda-duplex,id=sound0-codec0,bus=sound0.0,cad=0 -chardev spicevmc,id=charredir0,name=usbredir -device usb-redir,chardev=charredir0,id=redir0 -chardev spicevmc,id=charredir1,name=usbredir -device usb-redir,chardev=charredir1,id=redir1 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x8 -msg timestamp=on

When I set queues=251, then the guest can not start.
# virsh start rhel7.2
error: Failed to start domain rhel7.2
error: internal error: early end of file from monitor, possible problem: ((null):14879): SpiceDispatcher-ERROR **: dispatcher.c:260:dispatcher_init: socketpair failed Too many open files

when I set to 255, the guest can not start.
# virsh start rhel7.2
error: Failed to start domain rhel7.2
error: internal error: early end of file from monitor, possible problem: 2016-03-30T11:40:53.794624Z qemu-kvm: -netdev tap,fds=25:27:28:29:30:31:32:33:34:35:36:37:38:39:40:41:42:43:44:45:46:47:48:49:50:51:52:53:54:55:56:57:58:59:60:61:62:63:64:65:66:67:68:69:70:71:72:73:74:75:76:77:78:79:80:81:82:83:84:85:86:87:88:89:90:91:92:93:94:95:96:97:98:99:100:101:102:103:104:105:106:107:108:109:110:111:112:113:114:115:116:117:118:119:120:121:122:123:124:125:126:127:128:129:130:131:132:133:134:135:136:137:138:139:140:141:142:143:144:145:146:147:148:149:150:151:152:153:154:155:156:157:158:159:160:161:162:163:164:165:166:167:168:169:170:171:172:173:174:175:176:177:178:180:179:181:182:183:184:185:186:187:188:189:190:191:193:192:194:195:196:197:198:199:200:201:202:203:204:205:206:207:208:209:210:211:212:213:214:215:216:218:217:219:220:221:222:223:224:225:226:227:228:229:230:231:232:233:234:235:236:237:238:239:240:241:242:243:244:245:246:247:248:249:250:251:252:253:254:255:256:257:258:259:260:261:262:263:264:265:266:267:268:269:270:271:272:273:

The problem is too many open files: apparently libvirt needs to increase
the open file descriptor limit (also known as "nofile" limit) for qemu by the number of queues.

As comment 6 from Michael, clone this bug to 
Bug 1322661 - The max number of vCPUs KVM supports is 256 currently, so libvirt needs to increase the open file descriptor limit

And still need the document modification.

Thanks for pointing out the problem Yalan!

Is this change what you had in mind?

http://jenkinscat.gsslab.pnq.redhat.com:8080/job/doc-Red_Hat_Enterprise_Linux-7-Virtualization_Tuning_and_Optimization_Guide%20%28html-single%29/lastStableBuild/artifact/tmp/en-US/html-single/index.html#sect-Virtualization_Tuning_Optimization_Guide-Networking-Multi-queue_virtio-net

Also, would this also be relevant for RHEL6 documentation, or should the limit stay at 8 there?

http://jenkinscat.gsslab.pnq.redhat.com:8080/job/doc-Red_Hat_Enterprise_Linux-6-Virtualization_Tuning_and_Optimization_Guide%20%28html-single%29/lastStableBuild/artifact/tmp/en-US/html-single/index.html#sect-Virtualization_Tuning_Optimization_Guide-Networking-Multi-queue_virtio-net

Suggest to remove the the content "5.5. Network Tuning Techniques" from RHEL6 documentation as "Multi-Queue virtio-net" is not supported on RHEL6 host.
I have tried on RHEL6 host, the attribute "queues" can not even be saved.

queues
    The optional queues attribute controls the number of queues to be used for either Multiqueue virtio-net or vhost-user network interfaces. Use of multiple packet processing queues requires the interface having the <model type='virtio'/> element......virtio-net since 1.0.6 (QEMU and KVM only) vhost-user since 1.2.17 (QEMU and KVM only) ----from libvirt.org

And the base code of qemu for RHEL6 is 0.10.2, so multi-queue is not support on RHEL6 host.

Thanks for the feedback Yalan,

I have removed the Network Tuning Techniques section from the book:

http://jenkinscat.gsslab.pnq.redhat.com:8080/job/doc-Red_Hat_Enterprise_Linux-6-Virtualization_Tuning_and_Optimization_Guide%20%28html-single%29/lastStableBuild/artifact/tmp/en-US/html-single/index.html#chap-Virtualization_Tuning_Optimization_Guide-Networking

Moving this BZ to VERIFIED, please let me know if any other modifications are necessary.

The changes based on this BZ have gone live with the release of RHEL 7.3:

https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/Virtualization_Tuning_and_Optimization_Guide/chap-Virtualization_Tuning_Optimization_Guide-Networking.html

I will be closing the bug, feel free to reopen if deemed necessary.