Ray Lee wrote:
quoted text
> On 7/24/07, david@lang.hm <david@lang.hm> wrote:
> > by the way, I've also seen comments on the Postgres performance mailing
> > list about how slow linux is compared to other OS's in pulling data back
> > in that's been pushed out to swap (not a factor on dedicated database
> > machines, but a big factor on multi-purpose machines)
>
> Yeah, akpm and... one of the usual suspects, had mentioned something
> such as 2.6 is half the speed of 2.4 for swapin. (Let's see if I can
> find a reference for that, it's been a year or more...) Okay,
> misremembered. Swap in is half the speed of swap out (
> http://lkml.org/lkml/2007/1/22/173 ). Al Boldi (added to the CC:, poor
> sod), is the one who knows how to measure that, I'm guessing.
>
> Al? How are you coming up with those figures? I'm interested in
> reproducing it. It could be due to something stupid, such as the VM
> faulting things out in reverse order or something...

Thanks for asking.  I'm rather surprised why nobody's noticing any of this 
slowdown.  To be fair, it's not really a regression, on the contrary, 2.4 is 
lot worse wrt swapin and swapout, and Rik van Riel even considers a 50% 
swapin slowdown wrt swapout something like better than expected (see thread 
'[RFC] kswapd: Kernel Swapper performance').  He probably meant random 
swapin, which seems to offer a 4x slowdown.

There are two ways to reproduce this:

1. swsusp to disk reports ~44mb/s swapout, and ~25mb/s swapin during resume

2. tmpfs swapout is superfast, whereas swapin is really slow
(see thread '[PATCH] free swap space when (re)activating page')

Here is an excerpt from that thread (note machine config in first line):

============================================
 RAM 512mb , SWAP 1G
 #mount -t tmpfs -o size=1G none /dev/shm
 #time cat /dev/full > /dev/shm/x.dmp
 15sec
 #time cat /dev/shm/x.dmp > /dev/null
 58sec
 #time cat /dev/shm/x.dmp > /dev/null
 72sec
 #time cat /dev/shm/x.dmp > /dev/null
 85sec
 #time cat /dev/shm/x.dmp > /dev/null
 93sec
 #time cat /dev/shm/x.dmp > /dev/null
 99sec
============================================

As you can see, swapout is running full wirespeed, whereas swapin not only is 
4x slower, but increasingly gets the VM tangled up to end at a ~6x slowdown.

So again, I'm really surprised people haven't noticed.


Thanks!

--
Al

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