Hi Johannes, referring to an old kernel bugzilla issue: https://bugzilla.kernel.org/show_bug.cgi?id=64121 Am 01.11.2013 um 19:43 wrote Johannes Weiner: > It is a combination of two separate things on these setups. > > Traditionally, only lowmem is considered dirtyable so that dirty pages > don't scale with highmem and the kernel doesn't overburden itself with > lowmem pressure from buffers etc. This is purely about accounting. > > My patches on the other hand were about dirty page placement and > avoiding writeback from page reclaim: by subtracting the watermark and > the lowmem reserve (memory not available for user memory / cache) from > each zone's dirtyable memory, we make sure that the zone can always be > rebalanced without writeback. > > The problem now is that the lowmem reserves scale with highmem and > there is a point where they entirely overshadow the Normal zone. This > means that no page cache at all is allowed in lowmem. Combine this > with how dirtyable memory excludes highmem, and the sum of all > dirtyable memory is nil. This effectively disables the writeback > cache. > > I figure if anything should be fixed it should be the full exclusion > of highmem from dirtyable memory and find a better way to calculate a > minimum. recently we've updated our production mail server from 3.14.69 to 3.14.73 and it worked fine for a few days. When the box is really busy (=incoming malware via email), the I/O speed drops to crawl, write speed is about 5 MB/s on Intel SSDs. Yikes. The box has 16GB RAM, so it should be a safe HIGHMEM configuration. Downgrading to 3.14.69 or booting with "mem=15000M" works. I've tested both approaches and the box was stable. Booting 3.14.73 again triggered the problem within minutes. Clearly something with the automatic calculation of the lowmem reserve crossed a tipping point again, even with the previously considered safe amount of 16GB RAM for HIGHMEM configs. I don't see anything obvious in the changelogs from 3.14.69 to 3.14.73, but I might have missed it. > HOWEVER, > > the lowmem reserve is highmem/32 per default. With a Normal zone of > around 900M, this requires 28G+ worth of HighMem to eclipse lowmem > entirely. This is almost double of what you consider still okay... is there a way to read out the calculated lowmem reserve via /proc? It might be interesting the see the lowmem reserve when booted with mem=15000M or kernel 3.14.69 for comparison. Do you think it might be worth tinkering with "lowmem_reserve_ratio"? /proc/meminfo from the box using "mem=15000M" + kernel 3.14.73: MemTotal: 15001512 kB HighTotal: 14219160 kB HighFree: 9468936 kB LowTotal: 782352 kB LowFree: 117696 kB Slab: 430612 kB SReclaimable: 416752 kB SUnreclaim: 13860 kB /proc/meminfo from a similar machine with 16GB RAM + kernel 3.14.73: (though that machine is just a firewall, so no real disk I/O) MemTotal: 16407652 kB HighTotal: 15636376 kB HighFree: 14415472 kB LowTotal: 771276 kB LowFree: 562852 kB Slab: 34712 kB SReclaimable: 20888 kB SUnreclaim: 13824 kB Any help is appreciated, Thomas -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>