Rik, Satoru, Do you have any comments?
Seiji > -----Original Message----- > From: linux-kernel-ow...@vger.kernel.org > [mailto:linux-kernel-ow...@vger.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@vger.kernel.org; > linux...@kvack.org; lwood...@redhat.com; Seiji Aguchi; > a...@linux-foundation.org; hu...@google.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 majord...@vger.kernel.org More > majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
