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Subject: Re: [PATCH 5/5] hugetlbfs: Limit wait time when trying to share huge
 PMD
To:     Mike Kravetz <mike.kravetz@oracle.com>,
        Matthew Wilcox <willy@infradead.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Will Deacon <will.deacon@arm.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org, Davidlohr Bueso <dave@stgolabs.net>
References: <20190911150537.19527-1-longman@redhat.com>
 <20190911150537.19527-6-longman@redhat.com>
 <20190911151451.GH29434@bombadil.infradead.org>
 <19d9ea18-bd20-e02f-c1de-70e7322f5f22@redhat.com>
 <40a511a4-5771-f9a9-40b6-64e39478bbcb@oracle.com>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <5229662c-d709-7aca-be4c-53dea1a49fda@redhat.com>
Date:   Wed, 11 Sep 2019 18:15:26 +0100
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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.

Cheers,
Longman