From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 20BE4C38A2D for ; Tue, 25 Oct 2022 09:21:00 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1onFft-00043y-NC; Tue, 25 Oct 2022 04:52:42 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1onFPu-0001Vn-By for qemu-devel@nongnu.org; Tue, 25 Oct 2022 04:36:14 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1onFPr-0005Ax-IR for qemu-devel@nongnu.org; Tue, 25 Oct 2022 04:36:10 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1666686963; h=from:from:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:in-reply-to:in-reply-to: references:references; bh=Xi7S0NmttiPYdWjaTeXnGJbyYb/rStHUQiAyAnoBFco=; b=eSDAJ79uXwLThfiLl6IQebMbuECXioMCPa9iOo1SFK5W1nbUuUUj8UTZIsM1Z9IlaGRmLD VrtF7NO+naT7a5hg5NOIPj2ccqPeTALGKV0DEa4Yums02oZNrHwmNMhe43sgPw8udo4RRj Js7w6tKjiO5Eo642maGuldd8ErYIfNE= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-148-KkKJmiZ9Mw2OUUDX4WkP5w-1; Tue, 25 Oct 2022 04:35:59 -0400 X-MC-Unique: KkKJmiZ9Mw2OUUDX4WkP5w-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.rdu2.redhat.com [10.11.54.8]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 6F63A185A7A9 for ; Tue, 25 Oct 2022 08:35:59 +0000 (UTC) Received: from redhat.com (unknown [10.33.36.148]) by smtp.corp.redhat.com (Postfix) with ESMTPS id B34A5C15BAB; Tue, 25 Oct 2022 08:35:58 +0000 (UTC) Date: Tue, 25 Oct 2022 09:35:55 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Leonardo Bras Cc: Juan Quintela , "Dr. David Alan Gilbert" , Peter Xu , qemu-devel@nongnu.org Subject: Re: [RFC PATCH 4/4] migration/multifd/zero-copy: Flush only the LRU half of the header array Message-ID: References: <20221025044730.319941-1-leobras@redhat.com> <20221025044730.319941-5-leobras@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20221025044730.319941-5-leobras@redhat.com> User-Agent: Mutt/2.2.7 (2022-08-07) X-Scanned-By: MIMEDefang 3.1 on 10.11.54.8 Received-SPF: pass client-ip=170.10.133.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -25 X-Spam_score: -2.6 X-Spam_bar: -- X-Spam_report: (-2.6 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.503, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Sender: "Qemu-devel" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Tue, Oct 25, 2022 at 01:47:31AM -0300, Leonardo Bras wrote: > Zero-copy multifd migration sends both the header and the memory pages in a > single syscall. Since it's necessary to flush before reusing the header, a > header array was implemented, so each write call uses a different > array, and flushing only take place after all headers have been used, > meaning 1 flush for each N writes. > > This method has a bottleneck, though: After the last write, a flush will > have to wait for all writes to finish, which will be a lot, meaning the > recvmsg() syscall called in qio_channel_socket_flush() will be called a > lot. On top of that, it will create a time period when the I/O queue is > empty and nothing is getting send: between the flush and the next write. > > To avoid that, use qio_channel_flush()'s new max_pending parameter to wait > until at most half of the array is still in use. (i.e. the LRU half of the > array can be reused) > > Flushing for the LRU half of the array is much faster, since it does not > have to wait for the most recent writes to finish, making up for having > to flush twice per array. > > As a main benefit, this approach keeps the I/O queue from being empty while > there are still data to be sent, making it easier to keep the I/O maximum > throughput while consuming less cpu time. Doesn't this defeat the reason for adding the flush in the first place, which was to ensure that a migration iteration was fully sent before starting the next iteration over RAM ? If it is OK to only partially flush on each iteration, then why do we need to flush at all ? With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|