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SubjectRe: [PATCH 0/3] Unmapped page cache control (v5)
On 04/01/2011 09:17 AM, KOSAKI Motohiro wrote:
> Hi Christoph,
> Thanks, long explanation.
>> On Thu, 31 Mar 2011, KOSAKI Motohiro wrote:
>>> 1) zone reclaim doesn't work if the system has multiple node and the
>>> workload is file cache oriented (eg file server, web server, mail server, et al).
>>> because zone recliam make some much free pages than zone->pages_min and
>>> then new page cache request consume nearest node memory and then it
>>> bring next zone reclaim. Then, memory utilization is reduced and
>>> unnecessary LRU discard is increased dramatically.
>> That is only true if the webserver only allocates from a single node. If
>> the allocation load is balanced then it will be fine. It is useful to
>> reclaim pages from the node where we allocate memory since that keeps the
>> dataset node local.
> Why?
> Scheduler load balancing only consider cpu load. Then, usually memory
> pressure is no complete symmetric. That's the reason why we got the
> bug report periodically.

Agreed. As Christoph said if the allocation load is balanced it will be fine.
But I think it's not always true that the allocation load is balanced.

>>> But, I agree that now we have to concern slightly large VM change parhaps
>>> (or parhaps not). Ok, it's good opportunity to fill out some thing.
>>> Historically, Linux MM has "free memory are waste memory" policy, and It
>>> worked completely fine. But now we have a few exceptions.
>>> 1) RT, embedded and finance systems. They really hope to avoid reclaim
>>> latency (ie avoid foreground reclaim completely) and they can accept
>>> to make slightly much free pages before memory shortage.
>> In general we need a mechanism to ensure we can avoid reclaim during
>> critical sections of application. So some way to give some hints to the
>> machine to free up lots of memory (/proc/sys/vm/dropcaches is far too
>> drastic) may be useful.
> Exactly.
> I've heard multiple times this request from finance people. And I've also
> heared the same request from bullet train control software people recently.

I completely agree with you. I have both customers and they really need it
to make their critical section deterministic.


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