Re: [PATCH] cfq: Fix starvation of async writes in presence of heavy sync workload

[Vivek Goyal <vgoyal@redhat.com>]

On Mon, Jun 20, 2011 at 10:16:31AM -0400, Vivek Goyal wrote:
> In presence of heavy sync workload CFQ can starve asnc writes.
> If one launches multiple readers (say 16), then one can notice
> that CFQ can withhold dispatch of WRITEs for a very long time say
> 200 or 300 seconds.
> 
> Basically CFQ schedules an async queue but does not dispatch any
> writes because it is waiting for exisintng sync requests in queue to
> finish. While it is waiting, one or other reader gets queued up and
> preempts the async queue. So we did schedule the async queue but never
> dispatched anything from it. This can repeat for long time hence
> practically starving Writers.
> 
> This patch allows async queue to dispatch atleast 1 requeust once
> it gets scheduled and denies preemption if async queue has been
> waiting for sync requests to drain and has not been able to dispatch
> a request yet.
> 
> One concern with this fix is that how does it impact readers
> in presence of heavy writting going on.
> 
> I did a test where I launch firefox, load a website and close
> firefox and measure the time. I ran the test 3 times and took
> average.
> 
> - Vanilla kernel time ~= 1 minute 40 seconds
> - Patched kenrel time ~= 1 minute 35 seconds
> 

Forgot to mention that this was in the presence of a dd doing
writes.

dd if=/dev/zero of=zerofile bs=4K count=1M

Launching firefox takes around 25 seconds or so. Loading first
website takes a long time. I had a quick look at blktrace
and it I see for long time no reads are queued at all. Looks
like firefox has some dependency on some write to finish
before next read can be issued.

Thanks
Vivek