Re: Read starvation by sync writes

[Shaohua Li]

2012/12/11 Jan Kara <jack@xxxxxxx>:
>   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.

Thanks,
Shaohua
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