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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
