Re: [ck] Re: [PATCH] mm: Implement swap prefetching tweaks

[Jens Axboe <axboe@suse.de>]

On Tue, Mar 14 2006, Lee Revell wrote:
> On Tue, 2006-03-14 at 07:40 +0100, Mike Galbraith wrote:
> > On Sat, 2006-03-11 at 23:54 -0500, Lee Revell wrote:
> > > On Sat, 2006-03-11 at 08:51 +0100, Mike Galbraith wrote:
> > > > There used to be a pages in flight 'restrictor plate' in there that
> > > > would have probably helped this situation at least a little.  But in
> > > > any case, it sounds like you'll have to find a way to submit the IO in
> > > > itty bitty synchronous pieces. 
> > > 
> > > echo 64 > /sys/block/hd*/queue/max_sectors_kb
> > > 
> > > There is basically a straight linear relation between whatever you set
> > > this to and the maximum scheduling latency you see.  It was developed to
> > > solve the exact problem you are describing.
> > 
> > <head-scratching>
> > 
> > Is it possible that you mean pci latency?  I'm unable to measure any
> > scheduling latency > 5ms while pushing IO for all my little Barracuda
> > disk is worth.
> 
> It's only a big problem if LBA48 is in use which allows 32MB of IO to be
> in flight at once, this depends on the size of the drive.
> 
> What does that value default to?

Not quite true. Even if lba48 is active on the drive, we don't allow
more than 1MB per request. And nit picking a little, lba48 doesn't
always depend on the size of the drive, some drives smaller than 2^28
sectors also feature lba48 support.

-- 
Jens Axboe