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

[Robert Hancock <hancockr@shaw.ca>]

Al Boldi wrote:
> 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?

Depends if swap-in is doing any read-ahead. If it's reading one page at 
a time in from the disk then the performance will definitely suck 
because of all the overhead from the tiny I/O's. With random swap-in you 
then pay the horrible seek penalty for all the reads as well.

-- 
Robert Hancock      Saskatoon, SK, Canada
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