Re: [GIT PULL] Ext3 latency fixes

[Linus Torvalds <torvalds@linux-foundation.org>]

On Sat, 4 Apr 2009, Linus Torvalds wrote:

> 
> 
> On Sat, 4 Apr 2009, Jens Axboe wrote:
> > 
> > Big nack on this patch. Ted, this is EXACTLY where I told you we saw big
> > write regressions (sqlite performance drops by a factor of 4-5). Do a
> > git log on fs/buffer.c and see the original patch (which does what your
> > patch does) and the later revert. No idea why you are now suggestion
> > making that exact change?!
> 
> Jens, if I can re-create the 'fsync' times (I haven't yet), then the 
> default scheduler _will_ be switched to AS.

Btw, that patch is "obviously correct".

That write we're submitting is very much a synchronous write. After all, 
the code is literally

	ret = submit_bh(WRITE, bh);
	wait_on_buffer(bh);

and it just doesn't get any more synchronous than that. If we don't start 
the IO immediately (since we're _waiting_ for it immediately), we're 
broken. 

Now, if we need to fix some mysql throughput issue as a result, then I'd 
suggest that we look at whether "sync_dirty_buffer()" is sometimes called 
when it doesn't need to be od (b) whether perhaps the unplugging behavior 
is simply buggy in some other way.

But Ted's patch makes so much sense on a purely conceptual level, that 
when you look at the patch, you should almost not even need to see the 
performance numbers to know it's right. But together with the numbers Ted 
posted, it's a total no-brainer. CFQ is clearly broken here, and it's 
pretty clear that apparently CFQ has been tuned (improperly) purely for 
throughput.

			Linus