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Date: Fri, 15 Feb 2013 17:25:15 -0500 From: Rik van Riel <riel@...hat.com> To: Seiji Aguchi <seiji.aguchi@....com> CC: dormando <dormando@...ia.net>, Satoru Moriya <satoru.moriya@....com>, Randy Dunlap <rdunlap@...otime.net>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "linux-mm@...ck.org" <linux-mm@...ck.org>, "lwoodman@...hat.com" <lwoodman@...hat.com>, "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>, "hughd@...gle.com" <hughd@...gle.com> Subject: Re: extra free kbytes tunable On 02/15/2013 05:21 PM, Seiji Aguchi wrote: > Rik, Satoru, > > Do you have any comments? IIRC at the time the patch was rejected as too inelegant. However, nobody else seems to have come up with a better plan, and there are users in need of a fix for this problem. I would still like to see a fix for the problem merged upstream. >> -----Original Message----- >> From: linux-kernel-owner@...r.kernel.org [mailto:linux-kernel-owner@...r.kernel.org] On Behalf Of dormando >> Sent: Monday, February 11, 2013 9:01 PM >> To: Rik van Riel >> Cc: Randy Dunlap; Satoru Moriya; linux-kernel@...r.kernel.org; linux-mm@...ck.org; lwoodman@...hat.com; Seiji Aguchi; >> akpm@...ux-foundation.org; hughd@...gle.com >> Subject: extra free kbytes tunable >> >> Hi, >> >> As discussed in this thread: >> http://marc.info/?l=linux-mm&m=131490523222031&w=2 >> (with this cleanup as well: https://lkml.org/lkml/2011/9/2/225) >> >> A tunable was proposed to allow specifying the distance between pages_min and the low watermark before kswapd is kicked in to >> free up pages. I'd like to re-open this thread since the patch did not appear to go anywhere. >> >> We have a server workload wherein machines with 100G+ of "free" memory (used by page cache), scattered but frequent random io >> reads from 12+ SSD's, and 5gbps+ of internet traffic, will frequently hit direct reclaim in a few different ways. >> >> 1) It'll run into small amounts of reclaim randomly (a few hundred thousand). >> >> 2) A burst of reads or traffic can cause extra pressure, which kswapd occasionally responds to by freeing up 40g+ of the pagecache all >> at once >> (!) while pausing the system (Argh). >> >> 3) A blip in an upstream provider or failover from a peer causes the kernel to allocate massive amounts of memory for retransmission >> queues/etc, potentially along with buffered IO reads and (some, but not often a ton) of new allocations from an application. This >> paired with 2) can cause the box to stall for 15+ seconds. >> >> We're seeing this more in 3.4/3.5/3.6, saw it less in 2.6.38. Mass reclaims are more common in newer kernels, but reclaims still happen >> in all kernels without raising min_free_kbytes dramatically. >> >> I've found that setting "lowmem_reserve_ratio" to something like "1 1 32" >> (thus protecting the DMA32 zone) causes 2) to happen less often, and is generally less violent with 1). >> >> Setting min_free_kbytes to 15G or more, paired with the above, has been the best at mitigating the issue. This is simply trying to raise >> the distance between the min and low watermarks. With min_free_kbytes set to 15000000, that gives us a whopping 1.8G (!!!) of >> leeway before slamming into direct reclaim. >> >> So, this patch is unfortunate but wonderful at letting us reclaim 10G+ of otherwise lost memory. Could we please revisit it? >> >> I saw a lot of discussion on doing this automatically, or making kswapd more efficient to it, and I'd love to do that. Beyond making >> kswapd psychic I haven't seen any better options yet. >> >> The issue is more complex than simply having an application warn of an impending allocation, since this can happen via read load on >> disk or from kernel page allocations for the network, or a combination of the two (or three, if you add the app back in). >> >> It's going to get worse as we push machines with faster SSD's and bigger networks. I'm open to any ideas on how to make kswapd >> more efficient in our case, or really anything at all that works. >> >> I have more details, but cut it down as much as I could for this mail. >> >> Thanks, >> -Dormando >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More >> majordomo info at http://vger.kernel.org/majordomo-info.html >> Please read the FAQ at http://www.tux.org/lkml/ -- All rights reversed -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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