Ray Lee wrote:
> On 7/24/07, [email protected] <[email protected]> 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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