Re: fsync fixes for 2.4

[Andrea Arcangeli <andrea@suse.de>]

On Wed, Jul 17, 2002 at 10:44:18AM -0400, mgross wrote:
> On Monday 15 July 2002 06:07 am, Andrea Arcangeli wrote:
> > On Fri, Jul 12, 2002 at 02:52:11PM -0700, Griffiths, Richard A wrote:
> > > Mark is off climbing Mt. Hood, so he asked me to post the data on the
> > > fsync patch.
> 
> I was excited to report the significant improvement of 2.4.19rc1+fsync fix 
> over 2.4.18 and didn't realize that the improvement was not due to the fsync 
> patch.   I'm so glad Richard did a careful check, I was on my way out the 
> door for my vacation :)
> 
> I would like to know what's so good about 2.4.19rc1 that gives our block I/O 
> benchmark that significant improvement over 2.4.18.

that should be the effect of the first part of my vm updates that gone into 2.4.19pre.

> > > It appears from these results that there is no appreciable improvement
> > > using the
> > > fsync patch - there is a slight loss of top end on 4 adapters 1 drive.
> >
> > that's very much expected, as said with my new design by adding an
> > additional pass (third pass), I could remove the slight loss that I
> > expected from the simple patch that puts wait_on_buffer right in the
> > first pass.
> >
> > I mentioned this in my first email of the thread, so it looks all right.
> > For a rc2 the slight loss sounds like the simplest approch.
> >
> > If you care about it, with my new fsync accounting design we can fix it,
> > just let me know if you're interested about it. Personally I'm pretty
> > much fine with it this way too, as said in the first email if we block
> > it's likely bdflush is pumping the queue for us. the slowdown is most
> > probably due too early unplug of the queue generated by the blocking
> > points.
> 
> I don't care about the very slight (and possibly in the noise floor of our 
> test) reduction in throughput due to the fsync fix.  I think your's and 
> Andrews'  assertion of the bdflush / dirty page handling getting stopped up 
> is likely the problem preventing scaling to my personal goal of 250 to 
> 300MB/sec on our setup.

yep, should be. Actually running multiple fsyncs from multiple tasks
will kind of workaround the single threaded async flushing of 2.4.

> 
> Thanks,
> 
> Mark Gross
> PS I had a very nice time on mount hood.  I didn't make it to the top this 
> time too much snow had melted off the top of the thing to have a safe attempt 
> at the summit.  It was a guided (http://www.timberlinemtguides.com) 3 day 
> climb. 

:)

Andrea