Re: [RFC PATCH 0/3] block: Fix fsync slowness with CFQ cgroups

[Shaohua Li]

On Tue, 2011-06-28 at 04:17 +0800, Vivek Goyal wrote:
> Hi,
> 
> Konstantin reported that fsync is very slow with ext4 if fsyncing process
> is in a separate cgroup and one is using CFQ IO scheduler.
> 
> https://lkml.org/lkml/2011/6/23/269
> 
> Issue seems to be that fsync process is in a separate cgroup and journalling
> thread is in root cgroup. After every IO from fsync, CFQ idles on fysnc
> process queue waiting for more requests to come. But this process is now
> waiting for IO to finish from journaling thread. After waiting for 8ms
> fsync's queue gives way to jbd's queue. Then we start idling on jbd
> thread and new IO from fsync is sitting in a separate queue in a separate
> group.
> 
> Bottom line, that after every IO we end up idling on fysnc and jbd thread
> so much that if somebody is doing fsync after every 4K of IO, throughput
> nose dives.
> 
> Similar issue had issue come up with-in same cgroup also when "fsync"
> and "jbd" thread were being queued on differnt service trees and idling
> was killing. At that point of time two solutions were proposed. One
> from Jeff Moyer and one from Corrado Zoccolo.
> 
> Jeff came up with the idea of coming with block layer API to yield the
> queue if explicitly told by file system, hence cutting down on idling.
> 
> https://lkml.org/lkml/2010/7/2/277
> 
> Corrado, came up with a simpler approach of keeping jbd and fsync processes
> on same service tree by parsing RQ_NOIDLE flag. By queuing on same service
> tree, one queue preempts other queue hence cutting down on idling time.
> Upstream went ahead with simpler approach to fix the issue.
> 
> commit 749ef9f8423054e326f3a246327ed2db4b6d395f
> Author: Corrado Zoccolo <czoccolo@xxxxxxxxx>
> Date:   Mon Sep 20 15:24:50 2010 +0200
> 
>     cfq: improve fsync performance for small files
> 
> 
> Now with cgroups, same problem resurfaces but this time we can not queue
> both the processes on same service tree and take advantage of preemption
> as separate cgroups have separate service trees and both processes
> belong to separate cgroups. We do not allow cross cgroup preemption 
> as that wil break down the isolation between groups.
> 
> So this patch series resurrects Jeff's solution of file system specifying
> the IO dependencies between threads explicitly to the block layer/ioscheduler.
> One ioscheduler knows that current queue we are idling on is dependent on
> IO from some other queue, CFQ allows dispatch of requests from that other
> queue in the context of current active queue.
> 
> So if fysnc thread specifies the dependency on journalling thread, then
> when time slice of fsync thread is running, it allows dispatch from
> jbd in the time slice of fsync thread. Hence cutting down on idling.
> 
> This patch series seems to be working for me. I did testing for ext4 only.
> This series is based on for-3.1/core branch of Jen's block tree.
> Konstantin, can you please give it a try and see if it fixes your
> issue.
> 
> Any feedback on how to solve this issue is appreciated.
Hi Vivek,
can we introduce a group think time check in cfq? say in a group the
last queue is backed for the group and the queue is a non-idle queue, if
the group think time is big, we don't allow the group idle and preempt
could happen. The fsync thread is a non-idle queue with Corrado's patch,
this allows fast group switch.

Thanks,
Shaohua

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