Re: [PATCH 0/3] Make workingset detection logic memcg aware

From: Kamezawa Hiroyuki
Date: Sun Aug 09 2015 - 10:13:14 EST

On 2015/08/08 22:05, Vladimir Davydov wrote:
On Fri, Aug 07, 2015 at 10:38:16AM +0900, Kamezawa Hiroyuki wrote:
On 2015/08/06 17:59, Vladimir Davydov wrote:
On Wed, Aug 05, 2015 at 10:34:58AM +0900, Kamezawa Hiroyuki wrote:
I wonder, rather than collecting more data, rough calculation can help the situation.
for example,

(refault_disatance calculated in zone) * memcg_reclaim_ratio < memcg's active list

If one of per-zone calc or per-memcg calc returns true, refault should be true.

memcg_reclaim_ratio is the percentage of scan in a memcg against in a zone.

This particular formula wouldn't work I'm afraid. If there are two
isolated cgroups issuing local reclaim on the same zone, the refault
distance needed for activation would be reduced by half for no apparent

Hmm, you mean activation in memcg means activation in global LRU, and it's not a
valid reason. Current implementation does have the same issue, right ?

i.e. when a container has been hitting its limit for a while, and then, a file cache is
pushed out but came back soon, it can be easily activated.

I'd like to confirm what you want to do.

1) avoid activating a file cache when it was kicked out because of memcg's local limit.

No, that's not what I want. I want pages of the workingset to get
activated on refault no matter if they were evicted on global memory
pressure or due to hitting a memory cgroup limit.


2) maintain acitve/inactive ratio in memcg properly as global LRU does.
3) reclaim shadow entry at proper timing.

All ? hmm. It seems that mixture of record of global memory pressure and of local memory
pressure is just wrong.

What makes you think so? An example of misbehavior caused by this would
be nice to have.

By design, memcg's LRU aging logic is independent from global memory allocation/pressure.

Assume there are 4 containers(using much page-cache) with 1GB limit on 4GB server,
# contaienr A workingset=600M limit=1G (sleepy)
# contaienr B workingset=300M limit=1G (work often)
# container C workingset=500M limit=1G (work slowly)
# container D workingset=1.2G limit=1G (work hard)
container D can drive the zone's distance counter because of local memory reclaim.
If active/inactive = 1:1, container D page can be activated.
At kswapd(global reclaim) runs, all container's LRU will rotate.

Possibility of refault in A, B, C is reduced by conainer D's counter updates.

But yes, some _real_ test are required.

Now, the record is
ããããeviction | node | zone | 2bit.

How about changing this as

0 |eviction | node | zone | 2bit
1 |eviction | memcgid | 2bit

Assume each memcg has an eviction counter, which ignoring node/zone.
i.e. memcg local reclaim happens against memcg not against zone.

At page-in,
if (the 1st bit is 0)
compare eviction counter with zone's counter and activate the page if needed.
else if (the 1st bit is 1)
compare eviction counter with the memcg (if exists)

Having a single counter per memcg won't scale with the number of NUMA

It doesn't matter, we can use lazy counter like pcpu counter because it's not needed to be very accurate.

if (current memcg == recorded memcg && eviction distance is okay)
activate page.
At page-out
if (global memory pressure)
record eviction id with using zone's counter.
else if (memcg local memory pressure)
record eviction id with memcg's counter.

I don't understand how this is supposed to work when a memory cgroup
experiences both local and global pressure simultaneously.

I think updating global distance counter by local reclaim may update counter too much.
Above is to avoid updating zone's counter and keep memcg's LRU active/inactive balanced.

Also, what if a memory cgroup is protected by memory.low? Such a cgroup
may have all its pages in the active list, because it is never scanned.

If LRU never scanned, all file caches tend to be in never refaults.

This will affect the refault distance of other cgroups, making
activations unpredictable.


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