Hi Kame,

Sorry for the late response, I'm just back from vocation. : )

On Fri, Dec 28, 2012 at 8:39 AM, Kamezawa Hiroyuki
[off-list ref] wrote:

(2012/12/26 2:22), Sha Zhengju wrote:

quoted

From: Sha Zhengju <redacted>

Commit a8e7d49a(Fix race in create_empty_buffers() vs __set_page_dirty_buffers())
extracts TestSetPageDirty from __set_page_dirty and is far away from
account_page_dirtied. But it's better to make the two operations in one single
function to keep modular. So in order to avoid the potential race mentioned in
commit a8e7d49a, we can hold private_lock until __set_page_dirty completes.
There's no deadlock between ->private_lock and ->tree_lock after confirmation.
It's a prepare patch for following memcg dirty page accounting patches.


Here is some test numbers that before/after this patch:
Test steps(Mem-4g, ext4):
drop_cache; sync
fio (ioengine=sync/write/buffered/bs=4k/size=1g/numjobs=2/group_reporting/thread)

We test it for 10 times and get the average numbers:
Before:
write: io=2048.0MB, bw=254117KB/s, iops=63528.9 , runt=  8279msec
lat (usec): min=1 , max=742361 , avg=30.918, stdev=1601.02
After:
write: io=2048.0MB, bw=254044KB/s, iops=63510.3 , runt=  8274.4msec
lat (usec): min=1 , max=856333 , avg=31.043, stdev=1769.32

Note that the impact is little(<1%).


Signed-off-by: Sha Zhengju <redacted>
Reviewed-by: Michal Hocko <redacted>

Acked-by: KAMEZAWA Hiroyuki <redacted>

Hmm,..this change should be double-checked by vfs, I/O guys...

Now it seems they haven't paid attention here... I'll push it soon for
more review.

increasing hold time of mapping->private_lock doesn't affect performance ?

Yes, pointed by Fengguang in the previous round, mapping->private_lock and
mapping->tree_lock are often contented locks that in a dd testcase
they have the top
 #1 and #2 contention.
So the numbers above are trying to find the impaction of lock
contention by multiple
threads(numjobs=2) writing to the same file in parallel and it seems
the impact is
little (<1%).
I'm not sure if the test case is enough, any advice is welcomed! : )