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 C75ECCD6E4A for ; Tue, 2 Jun 2026 08:35:35 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists1p.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1wUKaM-0000gD-12; Tue, 02 Jun 2026 04:34:54 -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 1wUKaK-0000fz-69 for qemu-devel@nongnu.org; Tue, 02 Jun 2026 04:34:52 -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 1wUKaI-0000hj-HO for qemu-devel@nongnu.org; Tue, 02 Jun 2026 04:34:51 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1780389288; 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: in-reply-to:in-reply-to:references:references; bh=5pFa0FIkeV6ihl8xHC3tS17xLByKCnrUJUUSb9/GhuQ=; b=RUfFoCdoGwwOJjqfCCXGLxv1BMbRWj1VQsrIoHCVh/azHCfd9XE96k6b0oGVBZPLDOOeYK jPb+dewz+Ehb7KEYuzxlTngs4owjNLaTrKjCXIIgkiui4kA83Byq2DvBMdDsthAdreHdCw tVqVRTmeYWvXmLmwiogYkJNwqKE3bLk= 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-350-s7nbmAShM1icTDM7q_GDWg-1; Tue, 02 Jun 2026 04:34:47 -0400 X-MC-Unique: s7nbmAShM1icTDM7q_GDWg-1 X-Mimecast-MFC-AGG-ID: s7nbmAShM1icTDM7q_GDWg_1780389286 Received: from mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.93]) (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 33CB91956095; Tue, 2 Jun 2026 08:34:46 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.44.22.2]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id CEFF118004A3; Tue, 2 Jun 2026 08:34:45 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 5849621E6A01; Tue, 02 Jun 2026 10:34:43 +0200 (CEST) From: Markus Armbruster To: Bin Guo Cc: qemu-devel@nongnu.org Subject: Re: [PATCH] qobject/json-writer: preallocate output buffer In-Reply-To: <20260529023426.54680-1-guobin@linux.alibaba.com> (Bin Guo's message of "Fri, 29 May 2026 10:34:26 +0800") References: <20260529023426.54680-1-guobin@linux.alibaba.com> Date: Tue, 02 Jun 2026 10:34:43 +0200 Message-ID: <87ldcx5oik.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 Received-SPF: pass client-ip=170.10.133.124; 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_H5=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 Bin Guo writes: > 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. That's an extreme case. Most responses are *much* smaller. Still, starting with a larger buffer makes sense. > 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. 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. > The JSONWriter > is a short-lived object so the preallocation does not accumulate. > > Signed-off-by: Bin Guo > --- > qobject/json-writer.c | 5 ++++- > 1 file changed, 4 insertions(+), 1 deletion(-) > > diff --git a/qobject/json-writer.c b/qobject/json-writer.c > index aac2c6ab71..fb3f3f3e3c 100644 > --- a/qobject/json-writer.c > +++ b/qobject/json-writer.c > @@ -24,13 +24,16 @@ struct JSONWriter { > GByteArray *container_is_array; > }; > > +/* Covers most QMP responses without reallocation (one page) */ Covering most responses matters, one page does not. Suggest /* Should cover most QMP responses without reallocation */ > +#define JSON_WRITER_INITIAL_SIZE 4096 > + > JSONWriter *json_writer_new(bool pretty) > { > JSONWriter *writer = g_new(JSONWriter, 1); > > writer->pretty = pretty; > writer->need_comma = false; > - writer->contents = g_string_new(NULL); > + writer->contents = g_string_sized_new(JSON_WRITER_INITIAL_SIZE); > writer->container_is_array = g_byte_array_new(); > return writer; > } Consider tweaking the commit message and the comment to address my remarks. Reviewed-by: Markus Armbruster