Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1162382AbbKESRc (ORCPT ); Thu, 5 Nov 2015 13:17:32 -0500 Received: from mail-pa0-f51.google.com ([209.85.220.51]:32941 "EHLO mail-pa0-f51.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1162233AbbKESR2 (ORCPT ); Thu, 5 Nov 2015 13:17:28 -0500 Date: Thu, 5 Nov 2015 10:17:26 -0800 From: Shaohua Li To: Daniel Micay Cc: Andy Lutomirski , Minchan Kim , Hugh Dickins , Andrew Morton , Michael Kerrisk , Michal Hocko , "linux-mm@kvack.org" , KOSAKI Motohiro , "Kirill A. Shutemov" , Rik van Riel , Johannes Weiner , Linux API , Jason Evans , "linux-kernel@vger.kernel.org" , yalin wang , Mel Gorman Subject: Re: [PATCH v2 01/13] mm: support madvise(MADV_FREE) Message-ID: <20151105181726.GA63566@kernel.org> References: <1446600367-7976-1-git-send-email-minchan@kernel.org> <1446600367-7976-2-git-send-email-minchan@kernel.org> <56399CA5.8090101@gmail.com> <563A813B.9080903@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <563A813B.9080903@gmail.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2075 Lines: 39 On Wed, Nov 04, 2015 at 05:05:47PM -0500, Daniel Micay wrote: > > With enough pages at once, though, munmap would be fine, too. > > That implies lots of page faults and zeroing though. The zeroing alone > is a major performance issue. > > There are separate issues with munmap since it ends up resulting in a > lot more virtual memory fragmentation. It would help if the kernel used > first-best-fit for mmap instead of the current naive algorithm (bonus: > O(log n) worst-case, not O(n)). Since allocators like jemalloc and > PartitionAlloc want 2M aligned spans, mixing them with other allocators > can also accelerate the VM fragmentation caused by the dumb mmap > algorithm (i.e. they make a 2M aligned mapping, some other mmap user > does 4k, now there's a nearly 2M gap when the next 2M region is made and > the kernel keeps going rather than reusing it). Anyway, that's a totally > separate issue from this. Just felt like complaining :). > > > Maybe what's really needed is a MADV_FREE variant that takes an iovec. > > On an all-cores multithreaded mm, the TLB shootdown broadcast takes > > thousands of cycles on each core more or less regardless of how much > > of the TLB gets zapped. > > That would work very well. The allocator ends up having a sequence of > dirty spans that it needs to purge in one go. As long as purging is > fairly spread out, the cost of a single TLB shootdown isn't that bad. It > is extremely bad if it needs to do it over and over to purge a bunch of > ranges, which can happen if the memory has ended up being very, very > fragmentated despite the efforts to compact it (depends on what the > application ends up doing). I posted a patch doing exactly iovec madvise. Doesn't support MADV_FREE yet though, but should be easy to do it. http://marc.info/?l=linux-mm&m=144615663522661&w=2 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/