Re: Read starvation by sync writes

[Jan Kara <jack@suse.cz>]

On Thu 13-12-12 09:43:31, Shaohua Li wrote:
> 2012/12/12 Jan Kara <jack@suse.cz>:
> > On Wed 12-12-12 10:55:15, Shaohua Li wrote:
> >> 2012/12/11 Jan Kara <jack@suse.cz>:
> >> >   Hi,
> >> >
> >> >   I was looking into IO starvation problems where streaming sync writes (in
> >> > my case from kjournald but DIO would look the same) starve reads. This is
> >> > because reads happen in small chunks and until a request completes we don't
> >> > start reading further (reader reads lots of small files) while writers have
> >> > plenty of big requests to submit. Both processes end up fighting for IO
> >> > requests and writer writes nr_batching 512 KB requests while reader reads
> >> > just one 4 KB request or so. Here the effect is magnified by the fact that
> >> > the drive has relatively big queue depth so it usually takes longer than
> >> > BLK_BATCH_TIME to complete the read request. The net result is it takes
> >> > close to two minutes to read files that can be read under a second without
> >> > writer load. Without the big drive's queue depth, results are not ideal but
> >> > they are bearable - it takes about 20 seconds to do the reading. And for
> >> > comparison, when writer and reader are not competing for IO requests (as it
> >> > happens when writes are submitted as async), it takes about 2 seconds to
> >> > complete reading.
> >> >
> >> > Simple reproducer is:
> >> >
> >> > echo 3 >/proc/sys/vm/drop_caches
> >> > dd if=/dev/zero of=/tmp/f bs=1M count=10000 &
> >> > sleep 30
> >> > time cat /etc/* 2>&1 >/dev/null
> >> > killall dd
> >> > rm /tmp/f
> >> >
> >> >   The question is how can we fix this? Two quick hacks that come to my mind
> >> > are remove timeout from the batching logic (is it that important?) or
> >> > further separate request allocation logic so that reads have their own
> >> > request pool. More systematic fix would be to change request allocation
> >> > logic to always allow at least a fixed number of requests per IOC. What do
> >> > people think about this?
> >>
> >> As long as queue depth > workload iodepth, there is little we can do
> >> to prioritize tasks/IOC. Because throttling a task/IOC means queue
> >> will be idle. We don't want to idle a queue (especially for SSD), so
> >> we always push as more requests as possible to the queue, which
> >> will break any prioritization. As far as I know we always have such
> >> issue in CFQ for big queue depth disk.
> >   Yes, I understand that. But actually big queue depth on its own doesn't
> > make the problem really bad (at least for me). When the reader doesn't have
> > to wait for free IO requests, it progresses at a reasonable speed. What
> > makes it really bad is that big queue depth effectively disallows any use
> > of ioc_batching() mode for the reader and thus it blocks in request
> > allocation for every single read request unlike writer which always uses
> > its full batch (32 requests).
> 
> This can't explain why setting queue depth 1 makes the performance
> better.
  It does, when queue depth is small, reads are completed faster so reader
is able to submit more reads during one ioc_batching() period.

> In that case, write still get that number of requests, read will
> wait for a request. Anyway, try setting nr_request to a big number
> and check if performance is different.
  I have checked. Setting nr_requests to 100000 makes reader proceed at a
reasonable speed.

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR