From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pd0-f172.google.com (mail-pd0-f172.google.com [209.85.192.172]) by kanga.kvack.org (Postfix) with ESMTP id 429F46B0032 for ; Mon, 23 Feb 2015 17:47:01 -0500 (EST) Received: by pdbfp1 with SMTP id fp1so28681185pdb.9 for ; Mon, 23 Feb 2015 14:47:00 -0800 (PST) Received: from smtp2.provo.novell.com (smtp2.provo.novell.com. [137.65.250.81]) by mx.google.com with ESMTPS id f2si1190415pas.147.2015.02.23.14.46.59 for (version=TLSv1 cipher=RC4-SHA bits=128/128); Mon, 23 Feb 2015 14:47:00 -0800 (PST) Message-ID: <1424731603.6539.51.camel@stgolabs.net> Subject: Re: [RFC 0/6] the big khugepaged redesign From: Davidlohr Bueso Date: Mon, 23 Feb 2015 14:46:43 -0800 In-Reply-To: <1424696322-21952-1-git-send-email-vbabka@suse.cz> References: <1424696322-21952-1-git-send-email-vbabka@suse.cz> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Vlastimil Babka Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Hugh Dickins , Andrea Arcangeli , "Kirill A. Shutemov" , Rik van Riel , Mel Gorman , Michal Hocko , Ebru Akagunduz , Alex Thorlton , David Rientjes , Peter Zijlstra , Ingo Molnar 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, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org