Re: [PATCH 0/3] Unmapped page cache control (v5)

[Satoru Moriya <smoriya@redhat.com>]

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.

Thanks,
Satoru