Re: Plumbers 2018 - Performance and Scalability Microconference

[Davidlohr Bueso <dave@stgolabs.net>]

On Mon, 10 Sep 2018, Waiman Long wrote:

>On 09/08/2018 12:13 AM, John Hubbard wrote:
>>
>> Hi Daniel and all,
>>
>> I'm interested in the first 3 of those 4 topics, so if it doesn't conflict with HMM topics or
>> fix-gup-with-dma topics, I'd like to attend. GPUs generally need to access large chunks of
>> memory, and that includes migrating (dma-copying) pages around.
>>
>> So for example a multi-threaded migration of huge pages between normal RAM and GPU memory is an
>> intriguing direction (and I realize that it's a well-known topic, already). Doing that properly
>> (how many threads to use?) seems like it requires scheduler interaction.
>>
>> It's also interesting that there are two main huge page systems (THP and Hugetlbfs), and I sometimes
>> wonder the obvious thing to wonder: are these sufficiently different to warrant remaining separate,
>> long-term?  Yes, I realize they're quite different in some ways, but still, one wonders. :)
>
>One major difference between hugetlbfs and THP is that the former has to
>be explicitly managed by the applications that use it whereas the latter
>is done automatically without the applications being aware that THP is
>being used at all. Performance wise, THP may or may not increase
>application performance depending on the exact memory access pattern,
>though the chance is usually higher that an application will benefit
>than suffer from it.
>
>If an application know what it is doing, using hughtblfs can boost
>performance more than it can ever achieved by THP. Many large enterprise
>applications, like Oracle DB, are using hugetlbfs and explicitly disable
>THP. So unless THP can improve its performance to a level that is
>comparable to hugetlbfs, I won't see the later going away.

Yep, there are a few non-trivial workloads out there that flat out discourage
thp, ie: redis to avoid latency issues.

Thanks,
Davidlohr