Messages in this thread | | | From | Satoru Moriya <> | Date | Fri, 30 Mar 2012 18:44:27 -0400 | Subject | RE: [RFC][PATCH] avoid swapping out with swappiness==0 |
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Hello Kosaki-san,
On 03/07/2012 01:18 PM, Satoru Moriya wrote: > On 03/07/2012 12:19 PM, KOSAKI Motohiro wrote: >> Thank you. I brought back to memory it. Unfortunately DB folks are >> still mainly using RHEL5 generation distros. At that time, >> swapiness=0 doesn't mean disabling swap. >> >> They want, "don't swap as far as kernel has any file cache page". but >> linux don't have such feature. then they used swappiness for emulate >> it. So, I think this patch clearly make userland harm. Because of, we >> don't have an alternative way.
As I wrote in the previous mail(see below), with this patch the kernel begins to swap out when the sum of free pages and filebacked pages reduces less than watermark_high.
So the kernel reclaims pages like following.
nr_free + nr_filebacked >= watermark_high: reclaim only filebacked pages nr_free + nr_filebacked < watermark_high: reclaim only anonymous pages
Do you think this behavior satisfies DB users' requirement?
> If they expect the behavior that "don't swap as far as kernel has any > file cache page", this patch definitely helps them because if we set > swappiness==0, kernel does not swap out > *until* nr_free + nr_filebacked < high watermark in the zone. > It means kernel begins to swap out when nr_free + nr_filebacked > becomes less than high watermark. > > But, yes, this patch actually changes the behavior with swappiness==0 > and so it may make userland harm. > > How about introducing new value e.g -1 to avoid swap and maintain > compatibility?
Regards, Satoru
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