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The default value of bps_max is bps / 10 unless bps_max= was given on the command-line.
The following code explains why in util/throttle.c:throttle_fix_bucket():
/* The following is done to cope with the Linux CFQ block scheduler
* which regroup reads and writes by block of 100ms in the guest.
* When they are two process one making reads and one making writes cfq
* make a pattern looking like the following:
* WWWWWWWWWWWRRRRRRRRRRRRRRWWWWWWWWWWWWWwRRRRRRRRRRRRRRRRR
* Having a max burst value of 100ms of the average will help smooth the
* throttling
*/
min = bkt->avg / 10;
if (bkt->avg && !bkt->max) {
bkt->max = min;
}
I'm not sure if this default burst value is effective, but this behavior was intentionally put into the code by its author.
I guess the aim is to guarantee that QEMU will process one full 100 ms CFQ time slice, even if the limit value exactly matches the rate of I/O generated by the guest.
(In reply to Stefan Hajnoczi from comment #3)
> By the way, if there's a place where you'd like to see this documented just
> let me know. The QEMU man page doesn't document bps_max, etc.
Stefan, why not document it in the QEMU man page?
(In reply to Gu Nini from comment #4)
> (In reply to Stefan Hajnoczi from comment #3)
> > By the way, if there's a place where you'd like to see this documented just
> > let me know. The QEMU man page doesn't document bps_max, etc.
>
> Stefan, why not document it in the QEMU man page?
I have sent a QEMU patch to document all the -drive throttling options:
https://lists.nongnu.org/archive/html/qemu-devel/2017-02/msg04433.html
Description of problem: The throttling value of FOO is defined in qemu command,but show FOO_max lower than FOO after input info block. Version-Release number of selected component (if applicable): Host kernel:3.10.0-543.el7.ppc64le qemu: qemu-kvm-rhev-2.8.0-2.el7 slof: SLOF-20160223-6.gitdbbfda4.el7 Guest kernel:3.10.0-543.el7.ppc64 How reproducible: 100% Steps to Reproduce: 1.create two disk image eg: qemu-img create -f raw RHEL7-8380-20G.raw 20G qemu-img create -f raw RHEL7-8380-2-16G.raw 16G 2.boot a guest with bps,but not set the value of bps_max. /usr/libexec/qemu-kvm \ -name RHEL7-8380 \ -M pseries-rhel7.4.0 \ -m 64G \ -smp 4 \ -boot menu=on,order=c \ -device virtio-scsi-pci,id=virtio_scsi_pci0,bus=pci.0,addr=06 \ -drive file=/home/hyx/os/RHEL-7.3-20161019.0-Server-ppc64-dvd1.iso,if=none,media=cdrom,id=image0 \ -device scsi-cd,id=scsi-cd0,drive=image0,channel=0,scsi-id=0,lun=0,bootindex=1 \ -drive file=/home/hyx/image/RHEL7-8380-2-16G.raw,if=none,media=disk,format=raw,cache=none,bps=102400,iops=100,id=image2 \ -device scsi-hd,id=scsi-hd2,drive=image2,channel=0,scsi-id=0,lun=1 \ -drive file=/home/hyx/image/RHEL7-8380-20G.raw,if=none,media=disk,format=raw,id=image1 \ -device scsi-hd,id=scsi-hd1,drive=image1,channel=0,scsi-id=0,lun=2,bootindex=0 \ -netdev tap,id=hostnet0,script=/etc/qemu-ifup \ -device spapr-vlan,netdev=hostnet0,id=virtio-net-pci0,mac=70:e2:84:14:0e:03 \ -monitor stdio \ -serial unix:./sock0,server,nowait \ -qmp tcp:0:6666,server,nowait \ -device usb-tablet \ -vnc :0 3. info block (qemu) info block image0 (#block166): /home/hyx/os/RHEL-7.3-20161019.0-Server-ppc64-dvd1.iso (raw, read-only) Removable device: locked, tray closed Cache mode: writeback image2 (#block316): /home/hyx/image/RHEL7-8380-2-16G.raw (raw) Cache mode: writeback, direct I/O throttling: bps=102400 bps_rd=0 bps_wr=0 bps_max=10240 bps_rd_max=0 bps_wr_max=0 iops=100 iops_rd=0 iops_wr=0 iops_max=10 iops_rd_max=0 iops_wr_max=0 iops_size=0 group=image2 image1 (#block562): /home/hyx/image/RHEL7-8380-20G.raw (raw) Cache mode: writeback 4.set the value of bps_max same as the above info block show. /usr/libexec/qemu-kvm \ -name RHEL7-8380 \ -M pseries-rhel7.4.0 \ -m 64G \ -smp 4 \ -boot menu=on,order=c \ -device virtio-scsi-pci,id=virtio_scsi_pci0,bus=pci.0,addr=06 \ -drive file=/home/hyx/os/RHEL-7.3-20161019.0-Server-ppc64-dvd1.iso,if=none,media=cdrom,id=image0 \ -device scsi-cd,id=scsi-cd0,drive=image0,channel=0,scsi-id=0,lun=0,bootindex=1 \ -drive file=/home/hyx/image/RHEL7-8380-2-16G.raw,if=none,media=disk,format=raw,cache=none,bps=102400,bps_max=10240,iops=100,id=image2 \ -device scsi-hd,id=scsi-hd2,drive=image2,channel=0,scsi-id=0,lun=1 \ -drive file=/home/hyx/image/RHEL7-8380-20G.raw,if=none,media=disk,format=raw,id=image1 \ -device scsi-hd,id=scsi-hd1,drive=image1,channel=0,scsi-id=0,lun=2,bootindex=0 \ -netdev tap,id=hostnet0,script=/etc/qemu-ifup \ -device spapr-vlan,netdev=hostnet0,id=virtio-net-pci0,mac=70:e2:84:14:0e:03 \ -monitor stdio \ -serial unix:./sock0,server,nowait \ -qmp tcp:0:6666,server,nowait \ -device usb-tablet \ -vnc :0 5. boot guest Actual results: qemu-kvm:-drive file=/home/hyx/image/RHEL7-8380-2-16G.raw,if=none,media=disk,format=raw,cache=none,bps=102400,bps_max=10240,iops=100,id=image2: bps_max/iops_max cannot be lower than bps/iops. the bps_max is lower than bps. Expected results: The value of bps_max should be equal to bps, if the max value is not defined in cli. Additional info: