linux-kernel - Re: [PATCH 5/5] hugetlbfs: Limit wait time when trying to share huge PMD

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Message-Id: <C29A1EFA-148C-454E-91F1-93D5116FB640@lca.pw>
Date:   Wed, 11 Sep 2019 13:22:47 -0400
From:   Qian Cai <cai@....pw>
To:     Waiman Long <longman@...hat.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH 5/5] hugetlbfs: Limit wait time when trying to share huge
 PMD



> On Sep 11, 2019, at 1:15 PM, Waiman Long <longman@...hat.com> wrote:
> 
> On 9/11/19 6:03 PM, Mike Kravetz wrote:
>> On 9/11/19 8:44 AM, Waiman Long wrote:
>>> On 9/11/19 4:14 PM, Matthew Wilcox wrote:
>>>> On Wed, Sep 11, 2019 at 04:05:37PM +0100, Waiman Long wrote:
>>>>> When allocating a large amount of static hugepages (~500-1500GB) on a
>>>>> system with large number of CPUs (4, 8 or even 16 sockets), performance
>>>>> degradation (random multi-second delays) was observed when thousands
>>>>> of processes are trying to fault in the data into the huge pages. The
>>>>> likelihood of the delay increases with the number of sockets and hence
>>>>> the CPUs a system has.  This only happens in the initial setup phase
>>>>> and will be gone after all the necessary data are faulted in.
>>>> Can;t the application just specify MAP_POPULATE?
>>> Originally, I thought that this happened in the startup phase when the
>>> pages were faulted in. The problem persists after steady state had been
>>> reached though. Every time you have a new user process created, it will
>>> have its own page table.
>> This is still at fault time.  Although, for the particular application it
>> may be after the 'startup phase'.
>> 
>>>                         It is the sharing of the of huge page shared
>>> memory that is causing problem. Of course, it depends on how the
>>> application is written.
>> It may be the case that some applications would find the delays acceptable
>> for the benefit of shared pmds once they reach steady state.  As you say, of
>> course this depends on how the application is written.
>> 
>> I know that Oracle DB would not like it if PMD sharing is disabled for them.
>> Based on what I know of their model, all processes which share PMDs perform
>> faults (write or read) during the startup phase.  This is in environments as
>> big or bigger than you describe above.  I have never looked at/for delays in
>> these environments around pmd sharing (page faults), but that does not mean
>> they do not exist.  I will try to get the DB group to give me access to one
>> of their large environments for analysis.
>> 
>> We may want to consider making the timeout value and disable threshold user
>> configurable.
> 
> Making it configurable is certainly doable. They can be sysctl
> parameters so that the users can reenable PMD sharing by making those
> parameters larger.

It could be a Kconfig option, so people don’t need to change the setting every time
after reinstalling the system. There are times people don’t care too much
about those random multi-second delays. For example, running a debug kernel.