Re: [Qemu-devel] [Qemu-block] [PATCH 6/7] qemu-iotests: 141: reduce likelihood of race condition on systems with fast IO

[Sascha Silbe <silbe@linux.vnet.ibm.com>]

Dear Max,

Sascha Silbe <silbe@linux.vnet.ibm.com> writes:

> @Max: From a cursory glance at the code, maybe your 1 *byte* per second
> rate limit is being rounded down to 0 *blocks* per second, with 0
> meaning no limit? See e.g. mirror_set_speed(). Though I must admit I
> don't understand how speed=0 translates to unlimited (like
> qapi/block-core.json:block-job-set-speed says). My understanding of
> ratelimit_calculate_delay() is that speed=0 means "1 quantum per time
> slice", with time slice usually being 100ms; not sure about the
> quantum.

I think I've understood the issue now.

The backup, commit, mirror and stream actions operate in on full chunks,
with chunk size depending on the action and backing device. For
e.g. commit that means it always bursts at least 0.5MiB; that's where
the value the reference output comes from.

ratelimit_calculate_delay() lets through at least one burst per time
slice. This means the minimum rate is chunk size per time slice (always
100ms). So for commit and stream one will always get at least 5 MiB/s. A
surprisingly large value for something specified in bytes per second,
BTW. (I.e. it should probably be documented in qmp-commands.hx if it
stays this way).

On a busy or slow host, it may take the shell longer than the time slice
of 100ms to send the cancel command to qemu. When that happens,
additional chunks will get written before the job gets cancelled. That's
why I sometimes see 1 or even 1.5 MiB as offset, especially when running
CPU intensive workloads in parallel.

The best approach probably would be to fix up the rate limit code to
delay for multiple time slices if necessary. We should get rid of the
artificial BDRV_SECTOR_SIZE granularity at the same time, always using
bytes as the quantum unit.

Sascha
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