Linux-Fsdevel Archive on lore.kernel.org
 help / color / Atom feed
From: Qian Cai <cai@lca.pw>
To: Waiman Long <longman@redhat.com>
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>
Subject: Re: [PATCH 5/5] hugetlbfs: Limit wait time when trying to share huge PMD
Date: Wed, 11 Sep 2019 13:22:47 -0400
Message-ID: <C29A1EFA-148C-454E-91F1-93D5116FB640@lca.pw> (raw)
In-Reply-To: <5229662c-d709-7aca-be4c-53dea1a49fda@redhat.com>



> On Sep 11, 2019, at 1:15 PM, Waiman Long <longman@redhat.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.


  reply index

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-11 15:05 [PATCH 0/5] hugetlbfs: Disable PMD sharing for large systems Waiman Long
2019-09-11 15:05 ` [PATCH 1/5] locking/rwsem: Add down_write_timedlock() Waiman Long
2019-09-11 15:05 ` [PATCH 2/5] locking/rwsem: Enable timeout check when spinning on owner Waiman Long
2019-09-11 15:05 ` [PATCH 3/5] locking/osq: Allow early break from OSQ Waiman Long
2019-09-11 15:05 ` [PATCH 4/5] locking/rwsem: Enable timeout check when staying in the OSQ Waiman Long
2019-09-11 15:05 ` [PATCH 5/5] hugetlbfs: Limit wait time when trying to share huge PMD Waiman Long
2019-09-11 15:14   ` Matthew Wilcox
2019-09-11 15:44     ` Waiman Long
2019-09-11 17:03       ` Mike Kravetz
2019-09-11 17:15         ` Waiman Long
2019-09-11 17:22           ` Qian Cai [this message]
2019-09-11 17:28           ` Waiman Long
2019-09-11 16:01   ` Qian Cai
2019-09-11 16:34     ` Waiman Long
2019-09-11 19:42       ` Qian Cai
2019-09-11 20:54         ` Waiman Long
2019-09-11 21:57           ` Qian Cai
2019-09-11 19:57   ` Matthew Wilcox
2019-09-11 20:51     ` Waiman Long
2019-09-12  3:26   ` Mike Kravetz
2019-09-12  3:41     ` Matthew Wilcox
2019-09-12  4:40       ` Davidlohr Bueso
2019-09-16 13:53         ` Waiman Long
2019-09-12  9:06     ` Waiman Long
2019-09-12 16:43       ` Mike Kravetz
2019-09-13 18:23         ` Waiman Long
2019-09-13  1:50 ` [PATCH 0/5] hugetlbfs: Disable PMD sharing for large systems Dave Chinner
2019-09-25  8:35   ` Peter Zijlstra

Reply instructions:

You may reply publically to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=C29A1EFA-148C-454E-91F1-93D5116FB640@lca.pw \
    --to=cai@lca.pw \
    --cc=dave@stgolabs.net \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=longman@redhat.com \
    --cc=mike.kravetz@oracle.com \
    --cc=mingo@redhat.com \
    --cc=peterz@infradead.org \
    --cc=viro@zeniv.linux.org.uk \
    --cc=will.deacon@arm.com \
    --cc=willy@infradead.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-Fsdevel Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-fsdevel/0 linux-fsdevel/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-fsdevel linux-fsdevel/ https://lore.kernel.org/linux-fsdevel \
		linux-fsdevel@vger.kernel.org linux-fsdevel@archiver.kernel.org
	public-inbox-index linux-fsdevel

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-fsdevel


AGPL code for this site: git clone https://public-inbox.org/ public-inbox