Re: [performance bug] kernel building regression on 64 LCPUs machine

["Alex,Shi" <alex.shi@intel.com>]

On Wed, 2011-01-19 at 20:56 +0800, Jan Kara wrote:
> On Wed 19-01-11 10:03:26, Shaohua Li wrote:
> > add Jan and Theodore to the loop.
>   Thanks.
> 
> > On Wed, 2011-01-19 at 09:55 +0800, Shi, Alex wrote:
> > > Shaohua and I tested kernel building performance on latest kernel. and
> > > found it is drop about 15% on our 64 LCPUs NHM-EX machine on ext4 file
> > > system. We find this performance dropping is due to commit
> > > 749ef9f8423054e326f. If we revert this patch or just change the
> > > WRITE_SYNC back to WRITE in jbd2/commit.c file. the performance can be
> > > recovered.
> > >
> > > iostat report show with the commit, read request merge number increased
> > > and write request merge dropped. The total request size increased and
> > > queue length dropped. So we tested another patch: only change WRITE_SYNC
> > > to WRITE_SYNC_PLUG in jbd2/commit.c, but nothing effected.
> > since WRITE_SYNC_PLUG doesn't work, this isn't a simple no-write-merge issue.
> >
> > > we didn't test deadline IO mode, just test cfq. seems insert write
> > > request into sync queue effect much read performance, but we don't know
> > > details. What's your comments of this?
>   Indeed it seems that the optimization of the case where we wait for the
> transaction is negatively impacting the performance when we are not. Does
> patch below help for your load? It refines the logic when WRITE_SYNC
> is needed (of course, we should also test whether the patch works for fsync
> heavy loads as well).
>   The patch is mostly a proof of concept and only lightly tested so be
> careful...
> 

I tested the patch after remove t_synchronous_commit lines in
include/trace/events/jbd2.h on 2.6.38-rc1 kernel. 
but did not find clear improvement on kbuild.