Re: Can we disable transparent hugepages for lack of a legitimate use case please?

[Andres Freund <andres@anarazel.de>]

On 2015-09-10 18:45:06 +0200, Andrea Arcangeli wrote:
> On Mon, Aug 24, 2015 at 08:46:11PM +0000, James Hartshorn wrote:
> > Some more links to discussion
> > about THP: Postgresql https://lwn.net/Articles/591723/ Postgresql
> > http://www.postgresql.org/message-id/20120821131254.1415a545@jekyl.davidgould.org
> 
> "and my interpretation was that it was trying to create hugepages from
> scattered fragments"
> 
> This is a very old email, but I'm just taking it as an example because
> this has to be a compaction issue. If you run into very visible hangs
> that goes away by disabling THP, it can't be THP to blame. THP can
> increase the latency jitter during page faults (real time sensitive
> application could notice a 2MB clear_page vs a 4KB clear_page), but
> not in a way that hangs a system and becomes visible to the user.
> 
> It's just very early compaction code was too aggressive and it got
> fixed in the meanwhile.

There's still some slowdown (as of 4.0) in extreme postgres workloads
with THP and/or compaction enabled, but I've indeed not been able to
reproduce bad stalls or large (10%+) slowdowns with recent kernels.

Greetings,

Andres Freund

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