Re: How to make kernel block layer generate bigger request in the request queue?

[James Bottomley <James.Bottomley@suse.de>]

On Mon, 2010-04-12 at 14:26 -0400, Martin K. Petersen wrote:
> >>>>> "James" == James Bottomley <James.Bottomley@suse.de> writes:
> 
> >> Correct.  It's quite unlikely for pages to be contiguous so this is
> >> the best we can do.
> 
> James> Actually, average servers do about 50% contiguous on average
> James> since we changed the mm layer to allocate in ascending physical
> James> page order ...  this figure is highly sensitive to mm changes
> James> though, and can vary from release to release.
> 
> Interesting.  When did this happen?

The initial work was done by Bill Irwin, years ago.  For a while it was
good, but then after Mel Gorman did the page reclaim code, we became
highly sensitive to the reclaim algorithms for this, so it's fluctuated
a bit ever since.  Even with all this, the efficiency is highly
dependent on the amount of free memory: once the machine starts running
to exhaustion (excluding page cache, since that usually allocates
correctly to begin with) the contiguity really drops.

> Last time I gathered data on segment merge efficiency (1 year+ ago) I
> found that adjacent pages were quite rare for a normal fs type workload.
> Certainly not in the 50% ballpark.  I'll take another look when I have a
> moment...

I got 60% with an I/O bound test with about a gigabyte of free memory a
while ago (2.6.31, I think).  Even for machines approaching memory
starvation, 30% seems achievable.

James