Re: [PATCH 1/4] vmscan: delegate pageout io to flusher thread ifcurrent is kswapd

[Dave Chinner]

On Fri, Apr 16, 2010 at 10:50:02AM +0100, Alan Cox wrote:
> > No. If you are doing full disk seeks between random chunks, then you
> > still lose a large amount of throughput. e.g. if the seek time is
> > 10ms and your IO time is 10ms for each 4k page, then increasing the
> > size ito 64k makes it 10ms seek and 12ms for the IO. We might increase
> > throughput but we are still limited to 100 IOs per second. We've
> > gone from 400kB/s to 6MB/s, but that's still an order of magnitude
> > short of the 100MB/s full size IOs with little in way of seeks
> > between them will acheive on the same spindle...
> 
> The usual armwaving numbers for ops/sec for an ATA disk are in the 200
> ops/sec range so that seems horribly credible.

Yeah, in my experience 7200rpm SATA will get you 200 ops/s when you
are doing really small seeks as the typical minimum seek time is
around 4-5ms. Average seek time, however, is usually in the range of
10ms, because full head sweep + spindle rotation seeks take in the
order of 15ms.

Hence small random IO tends to result in seek times nearer the
average seek time than the minimum, so that's what i tend to use for
determining the number of ops/s a disk will sustain.

> But then I've never quite understood why our anonymous paging isn't
> sorting stuff as best it can and then using the drive as a log structure
> with in memory metadata so it can stream the pages onto disk. Read
> performance is goig to be similar (maybe better if you have a log tidy
> when idle), write ought to be far better.

Sounds like a worthy project for someone to sink their teeth into.
Lots of people would like to have a system that can page out at
hundreds of megabytes a second....

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx
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