Ray Lee wrote:

On 7/24/07, david@lang.hm [off-list ref] wrote:

quoted

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