lkml.org 
[lkml]   [2007]   [Jul]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: How can we make page replacement smarter (was: swap-prefetch)
Date
Chris Snook wrote:
> Al Boldi wrote:
> > Because it is hard to quantify the expected swap-in speed for random
> > pages, let's first tackle the swap-in of consecutive pages, which should
> > be at least as fast as swap-out. So again, why is swap-in so slow?
>
> If I'm writing 20 pages to swap, I can find a suitable chunk of swap and
> write them all in one place. If I'm reading 20 pages from swap, they
> could be anywhere. Also, writes get buffered at one or more layers of
> hardware.

Ok, this explains swap-in of random pages. Makes sense, but it doesn't
explain the awful tmpfs performance degradation of consecutive read-in runs
from swap, which should have at least stayed constant

> At best, reads can be read-ahead and cached, which is why
> sequential swap-in sucks less. On-demand reads are as expensive as I/O
> can get.

Which means that it should be at least as fast as swap-out, even faster
because write to disk is usually slower than read on modern disks. But
linux currently shows a distinct 2x slowdown for sequential swap-in wrt
swap-out. And to prove this point, just try suspend to disk where you can
see sequential swap-out being reported at about twice the speed of
sequential swap-in on resume. Why is that?


Thanks!

--
Al

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2007-07-28 13:17    [W:0.206 / U:4.980 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site