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SubjectRe: How can we make page replacement smarter (was: swap-prefetch)
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?



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