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

[Jeff Moyer]

Jan Kara <jack@xxxxxxx> writes:

> I'm not so happy with ext4 results. The difference between ext3 and ext4
> might be that amount of data written by kjournald in ext3 is considerably
> larger if it ends up pushing out data (because of data=ordered mode) as
> well. With ext4, all data are written by filemap_fdatawrite() from fsync
> because of delayed allocation. And thus maybe for ext4 WRITE_SYNC_PLUG
> is hurting us with your fast storage and small amount of written data? With
> WRITE_SYNC, data would be already on it's way to storage before we get to
> wait for them...

> Or it could be that we really send more data in WRITE mode rather than in
> WRITE_SYNC mode with the patch on ext4 (that should be verifiable with
> blktrace). But I wonder how that could happen...

It looks like this is the case, the I/O isn't coming down as
synchronous.  I'm seeing a lot of writes, very few write sync's, which
means that the write stream will be preempted by the incoming reads.

Time to audit that fsync path and make sure it's marked properly, I
guess.

Cheers,
Jeff
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