linux-kernel - Re: [RFC 0/6] the big khugepaged redesign

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <1424731603.6539.51.camel@stgolabs.net>
Date:	Mon, 23 Feb 2015 14:46:43 -0800
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Michal Hocko <mhocko@...e.cz>,
	Ebru Akagunduz <ebru.akagunduz@...il.com>,
	Alex Thorlton <athorlton@....com>,
	David Rientjes <rientjes@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC 0/6] the big khugepaged redesign

On Mon, 2015-02-23 at 13:58 +0100, Vlastimil Babka wrote:
> Recently, there was concern expressed (e.g. [1]) whether the quite aggressive
> THP allocation attempts on page faults are a good performance trade-off.
> 
> - THP allocations add to page fault latency, as high-order allocations are
>   notoriously expensive. Page allocation slowpath now does extra checks for
>   GFP_TRANSHUGE && !PF_KTHREAD to avoid the more expensive synchronous
>   compaction for user page faults. But even async compaction can be expensive.
> - During the first page fault in a 2MB range we cannot predict how much of the
>   range will be actually accessed - we can theoretically waste as much as 511
>   worth of pages [2]. Or, the pages in the range might be accessed from CPUs
>   from different NUMA nodes and while base pages could be all local, THP could
>   be remote to all but one CPU. The cost of remote accesses due to this false
>   sharing would be higher than any savings on the TLB.
> - The interaction with memcg are also problematic [1].
> 
> Now I don't have any hard data to show how big these problems are, and I
> expect we will discuss this on LSF/MM (and hope somebody has such data [3]).
> But it's certain that e.g. SAP recommends to disable THPs [4] for their apps
> for performance reasons.

There are plenty of examples of this, ie for Oracle:

https://blogs.oracle.com/linux/entry/performance_issues_with_transparent_huge
http://oracle-base.com/articles/linux/configuring-huge-pages-for-oracle-on-linux-64.php

Thanks,
Davidlohr

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/