From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx190.postini.com [74.125.245.190]) by kanga.kvack.org (Postfix) with SMTP id 8EB276B0070 for ; Mon, 7 Jan 2013 10:14:57 -0500 (EST) Message-ID: <50EAE66B.1020804@redhat.com> Date: Mon, 07 Jan 2013 10:14:51 -0500 From: Rik van Riel MIME-Version: 1.0 Subject: Re: [RFC]x86: clearing access bit don't flush tlb References: <20130107081213.GA21779@kernel.org> In-Reply-To: <20130107081213.GA21779@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Shaohua Li Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, akpm@linux-foundation.org, mingo@redhat.com, hpa@zytor.com On 01/07/2013 03:12 AM, Shaohua Li wrote: > > We use access bit to age a page at page reclaim. When clearing pte access bit, > we could skip tlb flush for the virtual address. The side effect is if the pte > is in tlb and pte access bit is unset, when cpu access the page again, cpu will > not set pte's access bit. So next time page reclaim can reclaim hot pages > wrongly, but this doesn't corrupt anything. And according to intel manual, tlb > has less than 1k entries, which coverers < 4M memory. In today's system, > several giga byte memory is normal. After page reclaim clears pte access bit > and before cpu access the page again, it's quite unlikely this page's pte is > still in TLB. Skiping the tlb flush for this case sounds ok to me. Agreed. In current systems, it can take a minute to write all of memory to disk, while context switch (natural TLB flush) times are in the dozens-of-millisecond timeframes. > And in some workloads, TLB flush overhead is very heavy. In my simple > multithread app with a lot of swap to several pcie SSD, removing the tlb flush > gives about 20% ~ 30% swapout speedup. > > Signed-off-by: Shaohua Li Reviewed-by: Rik van Riel -- All rights reversed -- 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