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 8D383CCFA05 for ; Thu, 6 Nov 2025 15:54:51 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1vH2JL-00009v-LP; Thu, 06 Nov 2025 10:54:07 -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 1vH2JJ-00009c-EG for qemu-devel@nongnu.org; Thu, 06 Nov 2025 10:54:05 -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 1vH2JH-0002rZ-4j for qemu-devel@nongnu.org; Thu, 06 Nov 2025 10:54:05 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1762444440; 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=Eao2U0Cg+WpYftE7DpjOP/WrvnfOylH9QJQOTLiLM9Y=; b=jTxjVXdUO62zqkozeh7jiV+XUK3vwjyhFcxoX9tDI+AlxFuBmJ5hKetuzbAf/BXCrCOrp9 LAB2xOpjB6RbQO8achZwvrME+ANGTaw9CJOafGGRwRy0YdLLxOsqxr+haewgjOk3MR+q3x zicU9KUjzNG1JnltqUT1L/uZ4IZooVM= Received: from mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-563-hYFk_J6oOX2mHHQsYVH8vw-1; Thu, 06 Nov 2025 10:53:55 -0500 X-MC-Unique: hYFk_J6oOX2mHHQsYVH8vw-1 X-Mimecast-MFC-AGG-ID: hYFk_J6oOX2mHHQsYVH8vw_1762444435 Received: from mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.111]) (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-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id DDC3218002E9; Thu, 6 Nov 2025 15:53:54 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.39]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 9A5A1180035F; Thu, 6 Nov 2025 15:53:53 +0000 (UTC) Date: Thu, 6 Nov 2025 15:53:50 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Jon Kohler Cc: Paolo Bonzini , qemu-devel@nongnu.org Subject: Re: [PATCH v2] util/oslib-posix: increase memprealloc thread count to 32 Message-ID: References: <20251106163143.4185468-1-jon@nutanix.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20251106163143.4185468-1-jon@nutanix.com> User-Agent: Mutt/2.2.14 (2025-02-20) X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 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: -23 X-Spam_score: -2.4 X-Spam_bar: -- X-Spam_report: (-2.4 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.271, 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_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_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 Thu, Nov 06, 2025 at 09:31:43AM -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 > --------------------------------------------- > > Note: Going above 32 threads appears to have diminishing returns at > the point where the memory bandwidth and context switching costs > appear to be a limiting factor to linear scaling. For posterity, on > the same system as above: > - 32 threads: 1m19s > - 48 threads: 1m4s > - 64 threads: 59s > - 240 threads: 50s > > Additional thread counts also get less interesting as the amount of > memory is to be preallocated is smaller. Putting that all together, > 32 threads appears to be a sane number with a solid speedup on fairly > modern hardware. To go faster, we'd either need to improve the hardware > (CPU/memory) itself or improve clear_pages_*() on the kernel side to > be more efficient. > > [1] 1e356fc14bea ("mem-prealloc: reduce large guest start-up and migration time.") > > Signed-off-by: Jon Kohler > --- > util/oslib-posix.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) Reviewed-by: Daniel P. Berrangé > > diff --git a/util/oslib-posix.c b/util/oslib-posix.c > index 3c14b72665..dc001da66d 100644 > --- a/util/oslib-posix.c > +++ b/util/oslib-posix.c > @@ -61,7 +61,7 @@ > #include "qemu/memalign.h" > #include "qemu/mmap-alloc.h" > > -#define MAX_MEM_PREALLOC_THREAD_COUNT 16 > +#define MAX_MEM_PREALLOC_THREAD_COUNT 32 > > struct MemsetThread; > > -- > 2.43.0 > > 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 :|