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Subject: Re: [PATCH 5/5] hugetlbfs: Limit wait time when trying to share huge
 PMD
From:   Qian Cai <cai@lca.pw>
In-Reply-To: <5229662c-d709-7aca-be4c-53dea1a49fda@redhat.com>
Date:   Wed, 11 Sep 2019 13:22:47 -0400
Cc:     Mike Kravetz <mike.kravetz@oracle.com>,
        Matthew Wilcox <willy@infradead.org>,
        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>
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Message-Id: <C29A1EFA-148C-454E-91F1-93D5116FB640@lca.pw>
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>
 <5229662c-d709-7aca-be4c-53dea1a49fda@redhat.com>
To:     Waiman Long <longman@redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Sep 11, 2019, at 1:15 PM, Waiman Long <longman@redhat.com> wrote:
>=20
> 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'.
>>=20
>>>                         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.
>>=20
>> 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.
>>=20
>> We may want to consider making the timeout value and disable =
threshold user
>> configurable.
>=20
> 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=E2=80=99t need to change the =
setting every time
after reinstalling the system. There are times people don=E2=80=99t care =
too much
about those random multi-second delays. For example, running a debug =
kernel.