Re: cfq-iosched preempt issues

[Jeff Moyer]

Vivek Goyal <vgoyal@xxxxxxxxxx> writes:

> On Wed, Mar 02, 2011 at 04:05:30PM -0500, Jeff Moyer wrote:
>> Vivek Goyal <vgoyal@xxxxxxxxxx> writes:
>> 
>> > On Wed, Mar 02, 2011 at 08:43:41PM +0800, Shaohua Li wrote:
>> >> queue preemption is good for some workloads and not for others. With commit
>> >> f8ae6e3eb825, the impact is amplified. I currently have two issues with it:
>> >> 1. In a multi-threaded workload, each thread runs a random read/write (for
>> >> example, mmap write) with iodepth 1. I found the queue depth gets smaller
>> >> with commit f8ae6e3eb825. The reason is write gets preempted, so more threads
>> >> are waitting for write, and on the other hand, there are less threads doing
>> >> read. This will make the queue depth small, so performance drops a little.
>> >> So in this case, speed up write can speed up read too, but we can't detect
>> >> it.
>> >> 2. cfq_may_dispatch doesn't limit queue depth if the queue is the sole queue.
>> >> What about if there are two queues, one sync and one async? If the sync queue's
>> >> think time is small, we can treat it as the sole queue, because the sync queue
>> >> will preempt async queue, so we don't need care about the async queue's latency.
>> >> The issue exists before, but f8ae6e3eb825 amplifies it. Below is a patch for it.
>> >> 
>> >> Any idea?
>> >
>> > CFQ is already very complicated, lets try to keep it simple. Because it
>> > is complicated, making it hierarchical for cgroup becomes even harder.
>> >
>> > IIUC, you are saying that cfqd->busy_queues check is not sufficient as
>> > it takes async queues also in account.
>> >
>> > So we can keep another count say, cfqd->busy_sync_queues and if there
>> > are no busy_sync_queues, allow unlimited depth and that should be
>> > a really simple few lines change.
>> 
>> That covers workload 2, but what about 1?  I'm really not sure what the
>> workload there is.
>
> But CFQ can't track that if reads are stuck behind peding writes. And the
> whole philosophy is that give READS the importance and not WRITES. So I
> am not sure what we can do about first case.

OK, I suspected it might be reads backed up behind writes, but wasn't
sure.  I agree that we can't tell that's happening, and it's less clear
whether we'd even want to do anything about it.

> If we are really worried about performance and willing to loose isolation
> in the process (read vs write isolation, or isolation across groups), then
> may be we can think of implementing another tunables say min_queue_depth.
> That tells CFQ that don't idle if you are not driving min_queue_depth.

Hm, I think that would break a lot of things.  ;-)

> But again, this should be backed by some real workloads.

I agree, and said as much in my initial response to Shaohua.

Cheers,
Jeff
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