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 X-Spam-Level: X-Spam-Status: No, score=-6.8 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0C57BC38A30 for ; Sun, 19 Apr 2020 16:24:25 +0000 (UTC) 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 mail.kernel.org (Postfix) with ESMTPS id C2062206A2 for ; Sun, 19 Apr 2020 16:24:24 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=crudebyte.com header.i=@crudebyte.com header.b="mIz+o3PD" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C2062206A2 Authentication-Results: mail.kernel.org; dmarc=fail (p=quarantine dis=none) header.from=crudebyte.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:43948 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jQCk3-00030B-VW for qemu-devel@archiver.kernel.org; Sun, 19 Apr 2020 12:24:24 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:41460 helo=eggs1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from <963b183c516011237108a0212402affd35f0b799@lizzy.crudebyte.com>) id 1jQCjI-00025J-Ko for qemu-devel@nongnu.org; Sun, 19 Apr 2020 12:23:36 -0400 Received: from Debian-exim by eggs1p.gnu.org with spam-scanned (Exim 4.90_1) (envelope-from <963b183c516011237108a0212402affd35f0b799@lizzy.crudebyte.com>) id 1jQCjI-0001S2-8M for qemu-devel@nongnu.org; Sun, 19 Apr 2020 12:23:36 -0400 Received: from lizzy.crudebyte.com ([91.194.90.13]:36621) by eggs1p.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from <963b183c516011237108a0212402affd35f0b799@lizzy.crudebyte.com>) id 1jQCjH-0000AX-P2 for qemu-devel@nongnu.org; Sun, 19 Apr 2020 12:23:35 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=crudebyte.com; s=lizzy; h=Cc:To:Subject:Date:From:References:In-Reply-To: Message-Id:Content-Type:Content-Transfer-Encoding:MIME-Version:Content-ID: Content-Description; bh=jPdUMVvPDb49VGrJsLB5Dy+OTfiFUvEBcazVO3auyKA=; b=mIz+o 3PDM0Xel6Awqkv1/reRMJZAQeH+9Mbs417FWxI4mKnPI+QcJI9W9dXQOfOvtbClRbcaSqh626hUQk DqepnK748Z6yiyhBsAyeNChBM9Cf5xwwtsJcICVMye+MGZc309qn3IXQ5/TLSIkDmYxZ8FU8QV+b/ i0JHXi5Xi8k6lQC35fgfDYf4n1m+V3CDBmtkg22JZbxR4xw23nCGSWq6L8Pif5v0ud8GaFVwdKcoE eY5fGbeNNPg/nOEleCC3WIdmh8BLI9rZLoQ3G/dYA8ElkL7f4wR94Bigdug0HWcBwwKog49So5ml+ +ug8IO6cZAQYFBlLXysO5xqwu6Fvg==; Message-Id: <963b183c516011237108a0212402affd35f0b799.1587309014.git.qemu_oss@crudebyte.com> In-Reply-To: References: From: Christian Schoenebeck Date: Sun, 19 Apr 2020 17:07:01 +0200 Subject: [PATCH v6 5/5] 9pfs: clarify latency of v9fs_co_run_in_worker() To: qemu-devel@nongnu.org Cc: Greg Kurz Received-SPF: none client-ip=91.194.90.13; envelope-from=963b183c516011237108a0212402affd35f0b799@lizzy.crudebyte.com; helo=lizzy.crudebyte.com X-detected-operating-system: by eggs1p.gnu.org: Linux 3.11 and newer X-Received-From: 91.194.90.13 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" As we just fixed a severe performance issue with Treaddir request handling, clarify this overall issue as a comment on v9fs_co_run_in_worker() with the intention to hopefully prevent such performance mistakes in future (and fixing other yet outstanding ones). Signed-off-by: Christian Schoenebeck --- hw/9pfs/coth.h | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) diff --git a/hw/9pfs/coth.h b/hw/9pfs/coth.h index a6851822d5..8b6f76840a 100644 --- a/hw/9pfs/coth.h +++ b/hw/9pfs/coth.h @@ -19,7 +19,7 @@ #include "qemu/coroutine.h" #include "9p.h" -/* +/** * we want to use bottom half because we want to make sure the below * sequence of events. * @@ -28,6 +28,16 @@ * 3. Enter the coroutine in the worker thread. * we cannot swap step 1 and 2, because that would imply worker thread * can enter coroutine while step1 is still running + * + * @b PERFORMANCE @b CONSIDERATIONS: As a rule of thumb, keep in mind + * that hopping between threads adds @b latency! So when handling a + * 9pfs request, avoid calling v9fs_co_run_in_worker() too often, because + * this might otherwise sum up to a significant, huge overall latency for + * providing the response for just a single request. For that reason it + * is highly recommended to fetch all data from fs driver with a single + * fs driver request on a background I/O thread (bottom half) in one rush + * first and then eventually assembling the final response from that data + * on main I/O thread (top half). */ #define v9fs_co_run_in_worker(code_block) \ do { \ -- 2.20.1