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 EB112CCFA07 for ; Wed, 5 Nov 2025 09:06:27 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1vGZT3-0005z0-8k; Wed, 05 Nov 2025 04:06:13 -0500 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 1vGZT0-0005yX-UQ for qemu-devel@nongnu.org; Wed, 05 Nov 2025 04:06:10 -0500 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 1vGZSy-0007k7-Q8 for qemu-devel@nongnu.org; Wed, 05 Nov 2025 04:06:10 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1762333567; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=JT3lY+gkcP7vA8o48OfuF4DrzIjw7+cLZeuS47LLdC4=; b=Rntkx5SnT1U+g4yc4SJW+whjoe6r/QBqZd1JW/C519SReH9//ZGMTEPCmInqR/Y1k577XN IUjzYJo0WG3pp/240CDhibx68gaVcgyKDCaEP4mgEpVElUkOfdTGxJGMnDfIdwmKEwkvoL Zrw47dbGBbzo/xs3hZ/JvnhjSlMagf4= Received: from mx-prod-mc-03.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-119-z4qS2qghPB2ZenxXVlko_Q-1; Wed, 05 Nov 2025 04:06:03 -0500 X-MC-Unique: z4qS2qghPB2ZenxXVlko_Q-1 X-Mimecast-MFC-AGG-ID: z4qS2qghPB2ZenxXVlko_Q_1762333562 Received: from mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.17]) (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-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id A37AE1956060; Wed, 5 Nov 2025 09:06:02 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.63]) by mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id C372F1956095; Wed, 5 Nov 2025 09:06:00 +0000 (UTC) Date: Wed, 5 Nov 2025 09:05:56 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Jon Kohler Cc: Paolo Bonzini , "qemu-devel@nongnu.org" , "ankur.a.arora@oracle.com" Subject: Re: [PATCH] util/oslib-posix: increase memprealloc thread count to 32 Message-ID: References: <20251103185750.1394036-1-jon@nutanix.com> <545E78A6-6013-45E1-9C3B-7D027FF12E00@nutanix.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <545E78A6-6013-45E1-9C3B-7D027FF12E00@nutanix.com> User-Agent: Mutt/2.2.14 (2025-02-20) X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 Received-SPF: pass client-ip=170.10.129.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -28 X-Spam_score: -2.9 X-Spam_bar: -- X-Spam_report: (-2.9 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.788, 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_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_PASS=-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?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Tue, Nov 04, 2025 at 08:33:05PM +0000, Jon Kohler wrote: > > > > On Nov 3, 2025, at 4:14 PM, Daniel P. Berrangé wrote: > > > > On Mon, Nov 03, 2025 at 11:57:50AM -0700, Jon Kohler wrote: > >> Increase MAX_MEM_PREALLOC_THREAD_COUNT from 16 to 32. This was last > >> touched in 2017 [1] and, since then, physical machine sizes and VMs > >> therein have continue to get even bigger, both on average and on the > >> extremes. > >> > >> For very large VMs, using 16 threads to preallocate memory can be a > >> non-trivial bottleneck during VM start-up and migration. Increasing > >> this limit to 32 threads reduces the time taken for these operations. > >> > >> Test results from quad socket Intel 8490H (4x 60 cores) show a fairly > >> linear gain of 50% with the 2x thread count increase. > >> > >> --------------------------------------------- > >> Idle Guest w/ 2M HugePages | Start-up time > >> --------------------------------------------- > >> 240 vCPU, 7.5TB (16 threads) | 2m41.955s > >> --------------------------------------------- > >> 240 vCPU, 7.5TB (32 threads) | 1m19.404s > >> --------------------------------------------- > > > > If we're configuring a guest with 240 vCPUs, then this implies the admin > > is expecting that the guest will consume upto 240 host CPUs worth of > > compute time. > > > > What is the purpose of limiting the number of prealloc threads to a > > value that is an order of magnitude less than the number of vCPUs the > > guest has been given ? > > Daniel - thanks for the quick review and thoughts here. > > I looked back through the original commits that led up to the current 16 > thread max, and it wasn’t immediately clear to me why we clamped it at > 16. Perhaps there was some other contention at the time. > > > Have you measured what startup time would look like with 240 prealloc > > threads ? Do we hit some scaling limit before that point making more > > prealloc threads counter-productive ? > > I have, and it isn’t wildly better, it comes down to about 50-ish seconds, > as you start running into practical limitations on the speed of memory, as > well as context switching if you’re doing other things on the host at the > same time. > > In playing around with some other values, here’s how they shake out: > 32 threads: 1m19s > 48 threads: 1m4s > 64 threads: 59s > … > 240 threads: 50s > > This also looks much less exciting when the amount of memory is > smaller. For smaller memory sizes (I’m testing with 7.5TB), anything > smaller than that gets less and less fun from a speedup perspective. > > Putting that all together, 32 seemed like a sane number with a solid > speedup on fairly modern hardware. Yep, that's useful background, I've no objectino to picking 32. Perhaps worth putting a bit more of this details into the commit message as background. 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 :|