From patchwork Mon Feb 19 14:54:59 2018 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Alberto Garcia X-Patchwork-Id: 10227967 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork.web.codeaurora.org (Postfix) with ESMTP id E1587602B7 for ; Mon, 19 Feb 2018 14:57:20 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id D219A23B34 for ; Mon, 19 Feb 2018 14:57:20 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id C66D82861C; Mon, 19 Feb 2018 14:57:20 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.8 required=2.0 tests=BAYES_00,DKIM_SIGNED, RCVD_IN_DNSWL_HI,T_DKIM_INVALID autolearn=ham version=3.3.1 Received: from lists.gnu.org (lists.gnu.org [208.118.235.17]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (No client certificate requested) by mail.wl.linuxfoundation.org (Postfix) with ESMTPS id B1E02285AF for ; Mon, 19 Feb 2018 14:57:18 +0000 (UTC) Received: from localhost ([::1]:50675 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1enmsX-0002uf-PP for patchwork-qemu-devel@patchwork.kernel.org; Mon, 19 Feb 2018 09:57:17 -0500 Received: from eggs.gnu.org ([2001:4830:134:3::10]:39845) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1enmrP-00025O-9u for qemu-devel@nongnu.org; Mon, 19 Feb 2018 09:56:08 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1enmrL-0000LM-21 for qemu-devel@nongnu.org; Mon, 19 Feb 2018 09:56:07 -0500 Received: from fanzine.igalia.com ([91.117.99.155]:35570) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_128_CBC_SHA1:16) (Exim 4.71) (envelope-from ) id 1enmrK-0008Os-MD; Mon, 19 Feb 2018 09:56:02 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=igalia.com; s=20170329; h=Message-Id:Date:Subject:Cc:To:From; bh=0OuKgf2PtiMIKHyikkieoGLszssy+hcxgq3xIeUh/gw=; b=BmOEDN2sFqjNguAalmQgPjXuYTR2pc24OwgoOs5IwlmB9/ortQuK/fZINPDb5XjJRrQUuozatVRwSXDqXWeNw4LVjuAHD4C1q/sKfxblEqdpbDKFsDzY6zkfQ2w26uIDK+giqOmXHsatOZAm7OVLRHG0YAvVJBcU0sykXqL9CagxxJD3Y3c4Ps2JoaZ3doO5Efh1nLs8JvBI7ZvmtDOZh6hVwqZSh7DYdafQXVAFoifFlY8h7hQ7eIEIz1d38fQzJDWjghG5zp7YcJHJHjZaq71p3mrugIDoSX1wGNOe8UwQ19CUlOSY46urhkz+AXElmirzn8UKm/pv03l4YeJYWA==; Received: from [194.100.51.2] (helo=perseus.local) by fanzine.igalia.com with esmtpsa (Cipher TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim) id 1enmqi-0004An-0a; Mon, 19 Feb 2018 15:55:24 +0100 Received: from berto by perseus.local with local (Exim 4.89) (envelope-from ) id 1enmqQ-00028B-EV; Mon, 19 Feb 2018 16:55:06 +0200 From: Alberto Garcia To: qemu-devel@nongnu.org Date: Mon, 19 Feb 2018 16:54:59 +0200 Message-Id: <20180219145459.8143-1-berto@igalia.com> X-Mailer: git-send-email 2.11.0 X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x (no timestamps) [generic] [fuzzy] X-Received-From: 91.117.99.155 Subject: [Qemu-devel] [PATCH] docs: document how to use the l2-cache-entry-size parameter X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , "Denis V . Lunev" , Alberto Garcia , qemu-block@nongnu.org, Max Reitz Errors-To: qemu-devel-bounces+patchwork-qemu-devel=patchwork.kernel.org@nongnu.org Sender: "Qemu-devel" X-Virus-Scanned: ClamAV using ClamSMTP This patch updates docs/qcow2-cache.txt explaining how to use the new l2-cache-entry-size parameter. Here's a more detailed technical description of this feature: https://lists.gnu.org/archive/html/qemu-block/2017-09/msg00635.html And here are some performance numbers: https://lists.gnu.org/archive/html/qemu-block/2017-12/msg00507.html Signed-off-by: Alberto Garcia Reviewed-by: Eric Blake --- docs/qcow2-cache.txt | 46 +++++++++++++++++++++++++++++++++++++++++++--- 1 file changed, 43 insertions(+), 3 deletions(-) diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt index b0571de4b8..170191a242 100644 --- a/docs/qcow2-cache.txt +++ b/docs/qcow2-cache.txt @@ -1,6 +1,6 @@ qcow2 L2/refcount cache configuration ===================================== -Copyright (C) 2015 Igalia, S.L. +Copyright (C) 2015, 2018 Igalia, S.L. Author: Alberto Garcia This work is licensed under the terms of the GNU GPL, version 2 or @@ -118,8 +118,8 @@ There are three options available, and all of them take bytes: There are two things that need to be taken into account: - - Both caches must have a size that is a multiple of the cluster - size. + - Both caches must have a size that is a multiple of the cluster size + (or the cache entry size: see "Using smaller cache sizes" below). - If you only set one of the options above, QEMU will automatically adjust the others so that the L2 cache is 4 times bigger than the @@ -143,6 +143,46 @@ much less often than the L2 cache, so it's perfectly reasonable to keep it small. +Using smaller cache entries +--------------------------- +The qcow2 L2 cache stores complete tables by default. This means that +if QEMU needs an entry from an L2 table then the whole table is read +from disk and is kept in the cache. If the cache is full then a +complete table needs to be evicted first. + +This can be inefficient with large cluster sizes since it results in +more disk I/O and wastes more cache memory. + +Since QEMU 2.12 you can change the size of the L2 cache entry and make +it smaller than the cluster size. This can be configured using the +"l2-cache-entry-size" parameter: + + -drive file=hd.qcow2,l2-cache-size=2097152,l2-cache-entry-size=4096 + +Some things to take into account: + + - The L2 cache entry size has the same restrictions as the cluster + size (power of two, at least 512 bytes). + + - Smaller entry sizes generally improve the cache efficiency and make + disk I/O faster. This is particularly true with solid state drives + so it's a good idea to reduce the entry size in those cases. With + rotating hard drives the situation is a bit more complicated so you + should test it first and stay with the default size if unsure. + + - Try different entry sizes to see which one gives faster performance + in your case. The block size of the host filesystem is generally a + good default (usually 4096 bytes in the case of ext4). + + - Only the L2 cache can be configured this way. The refcount cache + always uses the cluster size as the entry size. + + - If the L2 cache is big enough to hold all of the image's L2 tables + (as explained in the "Choosing the right cache sizes" section + earlier in this document) then none of this is necessary and you + can omit the "l2-cache-entry-size" parameter altogether. + + Reducing the memory usage ------------------------- It is possible to clean unused cache entries in order to reduce the