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 lists1p.gnu.org (lists1p.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 5F3E5CD6E64 for ; Wed, 3 Jun 2026 10:25:16 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists1p.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1wUim0-0000BB-4m; Wed, 03 Jun 2026 06:24:32 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists1p.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1wUilw-0000AW-Df for qemu-devel@nongnu.org; Wed, 03 Jun 2026 06:24:28 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1wUilu-0006jF-SS for qemu-devel@nongnu.org; Wed, 03 Jun 2026 06:24:28 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1780482265; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=H6nHcwHbFEgFNU9EyDFxom5nyPaw7sIvXcWBeVhgwqU=; b=DCe+iTj6g/pkCwYAQ6SAEYh37qBFNHJp5yziiZvAj3mD5JyuE2Tf8BuSBwXemDvWOhInkq z7uuipNITYr/z4Y/em9+HWnjPA6I2PheJo+ZgJsgItBUaz4oPrDloiZ/5m/gBRiMWIjRII w888SUJPKGCVH+m+8Q6ksmht5mpQ4hQ= Received: from mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-439-m8OMIHYYOAS6WUyNAAWYtg-1; Wed, 03 Jun 2026 06:24:11 -0400 X-MC-Unique: m8OMIHYYOAS6WUyNAAWYtg-1 X-Mimecast-MFC-AGG-ID: m8OMIHYYOAS6WUyNAAWYtg_1780482251 Received: from mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.4]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 7137E19560A5; Wed, 3 Jun 2026 10:24:10 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.44.22.2]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 1629630001A1; Wed, 3 Jun 2026 10:24:09 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 70FCA21E6A01; Wed, 03 Jun 2026 12:24:07 +0200 (CEST) From: Markus Armbruster To: Daniel P. =?utf-8?Q?Berrang=C3=A9?= Cc: Bin Guo , qemu-devel@nongnu.org Subject: Re: [PATCH] qobject/json-writer: preallocate output buffer In-Reply-To: ("Daniel P. =?utf-8?Q?Berrang?= =?utf-8?Q?=C3=A9=22's?= message of "Wed, 3 Jun 2026 10:52:24 +0100") References: <20260529023426.54680-1-guobin@linux.alibaba.com> <87ldcx5oik.fsf@pond.sub.org> Date: Wed, 03 Jun 2026 12:24:07 +0200 Message-ID: <878q8vly60.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 Received-SPF: pass client-ip=170.10.129.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: 8 X-Spam_score: 0.8 X-Spam_bar: / X-Spam_report: (0.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.445, 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_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_SBL_CSS=3.335, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development 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-bounces+qemu-devel=archiver.kernel.org@nongnu.org Daniel P. Berrang=C3=A9 writes: > On Tue, Jun 02, 2026 at 10:34:43AM +0200, Markus Armbruster wrote: >> Bin Guo writes: >>=20 >> > json_writer_new() creates the output GString with g_string_new(NULL), >> > which starts at the GLib default of 64 bytes. Serializing typical >> > QMP responses then requires multiple reallocations as the buffer >> > grows -- for query-qmp-schema the GString is reallocated 12+ times. >>=20 >> That's an extreme case. Most responses are *much* smaller. Still, >> starting with a larger buffer makes sense. >>=20 >> > Preallocate JSON_WRITER_INITIAL_SIZE (4096) bytes. This covers >> > most QMP responses without any reallocation. 4096 is one page on >> > most systems, which is efficient for the allocator. >>=20 >> I doubt "one page" matters. How many QMP commands get executed in >> practice? A couple of hundred during startup, then tens per second? >> Probably less than that. > > NB tens per second, repeated across possibly 100's or even 1000's of > VMs on the single host though. > > If we want an arbitrary moderately size buffer, one page feels like > a reasonable place to aim for I think 4KiB is a reasonable initial size regardless of the host's page size. That was my argument. If we care, we can instrument QEMU to gather response size statistics. [...]