Date:	Mon, 27 Jun 2011 16:17:41 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	linux-kernel@...r.kernel.org, jaxboe@...ionio.com,
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org
Cc:	vgoyal@...hat.com, khlebnikov@...nvz.org, jmoyer@...hat.com
Subject: [RFC PATCH 0/3] block: Fix fsync slowness with CFQ cgroups 

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@...il.com>
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.

Thanks
Vivek


Vivek Goyal (3):
  block: A new interface for specifying IO dependencing among tasks
  ext4: Explicitly specify fsync dependency on journaling thread
  ext3: Explicitly specify fsync dependency on journaling thread

 block/blk-core.c         |   42 ++++++++
 block/cfq-iosched.c      |  236 ++++++++++++++++++++++++++++++++++++++++++---
 block/elevator.c         |   16 +++
 fs/ext3/fsync.c          |    3 +
 fs/ext4/fsync.c          |    3 +
 include/linux/blkdev.h   |    8 ++-
 include/linux/elevator.h |    6 +
 7 files changed, 297 insertions(+), 17 deletions(-)

-- 
1.7.4.4

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