RE: extra free kbytes tunable

From: Satoru Moriya
Date: Fri Feb 15 2013 - 17:49:46 EST

On 02/15/2013 05:21 PM, Seiji Aguchi wrote:
> Rik, Satoru,
> Do you have any comments?
> Seiji

Hmm, this seems what we wanted to know in the previous thread.

Because extra_free_kbytes is quite simple and it fixes the problem,
it should be merged into upstream.


>> -----Original Message-----
>> From: linux-kernel-owner@xxxxxxxxxxxxxxx
>> [mailto:linux-kernel-owner@xxxxxxxxxxxxxxx] On Behalf Of dormando
>> Sent: Monday, February 11, 2013 9:01 PM
>> To: Rik van Riel
>> Cc: Randy Dunlap; Satoru Moriya; linux-kernel@xxxxxxxxxxxxxxx;
>> linux-mm@xxxxxxxxx; lwoodman@xxxxxxxxxx; Seiji Aguchi;
>> akpm@xxxxxxxxxxxxxxxxxxxx; hughd@xxxxxxxxxx
>> Subject: extra free kbytes tunable
>> Hi,
>> As discussed in this thread:
>> (with this cleanup as well:
>> 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
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