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

[Shaohua Li <shaohua.li@intel.com>]

On Thu, 2011-01-20 at 23:16 +0800, Vivek Goyal wrote:
> On Wed, Jan 19, 2011 at 10:03:26AM +0800, Shaohua Li wrote:
> > add Jan and Theodore to the loop.
> > 
> > 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.
> > 
> 
> Yep, it does sound like reduce write merging. But moving journal commits
> back to WRITE, then fsync performance will drop as there will be idling
> introduced between fsync thread and journalling thread. So that does
> not sound like a good idea either.
> 
> Secondly, in presence of mixed workload (some other sync read happening)
> WRITES can get less bandwidth and sync workload much more. So by 
> marking journal commits as WRITES you might increase the delay there
> in completion in presence of other sync workload.
> 
> So Jan Kara's approach makes sense that if somebody is waiting on
> commit then make it WRITE_SYNC otherwise make it WRITE. Not sure why
> did it not work for you. Is it possible to run some traces and do
> more debugging that figure out what's happening.
I'll debug and see if I can find anything.

Thanks,
Shaohua