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Re: [Qemu-devel] [Qemu-block] [PATCH] throttle-groups: fix hang when gro
From: |
Kashyap Chamarthy |
Subject: |
Re: [Qemu-devel] [Qemu-block] [PATCH] throttle-groups: fix hang when group member leaves |
Date: |
Fri, 13 Jul 2018 14:19:45 +0200 |
User-agent: |
Mutt/1.9.2 (2017-12-15) |
On Wed, Jul 04, 2018 at 03:54:10PM +0100, Stefan Hajnoczi wrote:
> Throttle groups consist of members sharing one throttling state
> (including bps/iops limits). Round-robin scheduling is used to ensure
> fairness. If a group member already has a timer pending then other
> groups members do not schedule their own timers. The next group member
> will have its turn when the existing timer expires.
>
> A hang may occur when a group member leaves while it had a timer
> scheduled. Although the code carefully removes the group member from
> the round-robin list, it does not schedule the next member. Therefore
> remaining members continue to wait for the removed member's timer to
> expire.
>
> This patch schedules the next request if a timer is pending.
> Unfortunately the actual bug is a race condition that I've been unable
> to capture in a test case.
>
> Sometimes drive2 hangs when drive1 is removed from the throttling group:
>
> $ qemu ... -drive
> if=none,id=drive1,cache=none,format=qcow2,file=data1.qcow2,iops=100,group=foo
> \
> -device virtio-blk-pci,id=virtio-blk-pci0,drive=drive1 \
> -drive
> if=none,id=drive2,cache=none,format=qcow2,file=data2.qcow2,iops=10,group=foo \
> -device virtio-blk-pci,id=virtio-blk-pci1,drive=drive2
> (guest-console1)# fio -filename /dev/vda 4k-seq-read.job
> (guest-console2)# fio -filename /dev/vdb 4k-seq-read.job
> (qmp) {"execute": "block_set_io_throttle", "arguments": {"device":
> "drive1","bps": 0,"bps_rd": 0,"bps_wr": 0,"iops": 0,"iops_rd": 0,"iops_wr":
> 0}}
Hi Stefan,
I realize you want to preserve the long lines to not break the JSON QMP
command. But, FWIW, you might want to format it using one of the
convenient websites: https://jsonformatter.org/
So your QMP command nicely wraps (for the 'cost' of 11 extra lines):
{
"execute": "block_set_io_throttle",
"arguments": {
"device": "drive1",
"bps": 0,
"bps_rd": 0,
"bps_wr": 0,
"iops": 0,
"iops_rd": 0,
"iops_wr": 0
}
}
[...]
--
/kashyap