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

From: Waiman Long
Date: Wed Sep 11 2019 - 13:28:44 EST

On 9/11/19 6:15 PM, Waiman Long 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.

I suspect that the customer's application may be generating a new
process with its own address space for each transaction. That will be
causing a lot of PMD sharing operations when hundreds of threads are
pounding it simultaneously. I had inserted some instrumentation code to
a test kernel that the customers used for testing, the number of
timeouts after a certain time went up more than 20k.

On the other hands, if the application is structured in such a way that
there is limited number of separate address spaces with worker threads
processing the transaction, PMD sharing will be less of a problem. It
will be hard to convince users to make such a structural changes to
their application.