Re: [PATCH 0/3 v5][RFC] ext3/4: enhance fsync performance when using CFQ

[Jeff Moyer <jmoyer@redhat.com>]

Christoph Hellwig <hch@infradead.org> writes:

> On Tue, Jun 22, 2010 at 05:34:59PM -0400, Jeff Moyer wrote:
>> Hi,
>> 
>> Running iozone with the fsync flag, or fs_mark, the performance of CFQ is
>> far worse than that of deadline for enterprise class storage when dealing
>> with file sizes of 8MB or less.  I used the following command line as a
>> representative test case:
>> 
>>   fs_mark -S 1 -D 10000 -N 100000 -d /mnt/test/fs_mark -s 65536 -t 1 -w 4096 -F
>> 
>> When run using the deadline I/O scheduler, an average of the first 5 numbers
>> will give you 448.4 files / second.  CFQ will yield only 106.7.  With
>> this patch series applied (and the two patches I sent yesterday), CFQ now
>> achieves 462.5 files / second.
>> 
>> This patch set is still an RFC.  I'd like to make it perform better when
>> there is a competing sequential reader present.  For now, I've addressed
>> the concerns voiced about the previous posting.
>
> What happened to the initial idea of just using the BIO_RW_META flag
> for log writes?  In the end log writes are the most important writes you
> have in a journaled filesystem, and they should not be effect to any
> kind of queue idling logic or other interruption.  Log I/O is usually
> very little (unless you use old XFS code with a worst-case directory
> manipulation workload), and very latency sensitive.

Vivek showed that starting firefox in the presence of a processing doing
fsyncs (using the RQ_META approach) took twice as long as without the
patch:
  http://lkml.org/lkml/2010/4/6/276

Cheers,
Jeff