[lkml]   [2018]   [Jul]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH] mm: thp: remove use_zero_page sysfs knob

On 7/23/18 2:33 PM, David Rientjes wrote:
> On Mon, 23 Jul 2018, David Rientjes wrote:
>>>> The huge zero page can be reclaimed under memory pressure and, if it is,
>>>> it is attempted to be allocted again with gfp flags that attempt memory
>>>> compaction that can become expensive. If we are constantly under memory
>>>> pressure, it gets freed and reallocated millions of times always trying to
>>>> compact memory both directly and by kicking kcompactd in the background.
>>>> It likely should also be per node.
>>> Have you benchmarked making the non-huge zero page per-node?
>> Not since we disable it :) I will, though. The more concerning issue for
>> us, modulo CVE-2017-1000405, is the cpu cost of constantly directly
>> compacting memory for allocating the hzp in real time after it has been
>> reclaimed. We've observed this happening tens or hundreds of thousands
>> of times on some systems. It will be 2MB per node on x86 if the data
>> suggests we should make it NUMA aware, I don't think the cost is too high
>> to leave it persistently available even under memory pressure if
>> use_zero_page is enabled.
> Measuring access latency to 4GB of memory on Naples I observe ~6.7%
> slower access latency intrasocket and ~14% slower intersocket.
> use_zero_page is currently a simple thp flag, meaning it rejects writes
> where val != !!val, so perhaps it would be best to overload it with
> additional options? I can imagine 0x2 defining persistent allocation so
> that the hzp is not freed when the refcount goes to 0 and 0x4 defining if
> the hzp should be per node. Implementing persistent allocation fixes our
> concern with it, so I'd like to start there. Comments?

Sounds worth trying to me :-)  It might be worth making it persistent by
default. Keeping 2MB memory unreclaimable sounds not harmful for the use
case which prefer to use THP.

 \ /
  Last update: 2018-07-23 23:50    [W:0.074 / U:4.104 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site