From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mx0b-001b2d01.pphosted.com (mx0b-001b2d01.pphosted.com [148.163.158.5]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 87D7B1F5FD for ; Wed, 2 Apr 2025 07:44:27 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=148.163.158.5 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1743579869; cv=none; b=c5RdV8/2SnyHHajSonyhopwEnGF/VA/aKKCXLiYP3apjJ/gu5arIR/zrUKb4WbXLg1eQ2A694/OjPMH7w8K6LiKtzqeLFONHtb8EaQ53Vssg4torUxQIeJU8OSzP8fpT399QjXGLH2YFJw331edyX/bQEm8jRI4XlBcmzb6ulz0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1743579869; c=relaxed/simple; bh=yjZZzDbFLgRCER335g45rHCkxAKw5okUXIMJri+Kffs=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=OPxRn0uOBs/YHkn4YZdf934H4X7q+anKUzsrvVRe7R8JJUDindslMDi+AuyFqG2iSx8Q0Z5nXK/CrMffvXC6onU4mZag1Ot5GcuRSdG00ahpWJmSCWA0HlN50A6kjnzXIVhfmTdCF11m2YQoNSYewHV5vHwYsNwnyhf4iju9eW4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.ibm.com; spf=pass smtp.mailfrom=linux.ibm.com; dkim=pass (2048-bit key) header.d=ibm.com header.i=@ibm.com header.b=tIArEPdS; arc=none smtp.client-ip=148.163.158.5 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.ibm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.ibm.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=ibm.com header.i=@ibm.com header.b="tIArEPdS" Received: from pps.filterd (m0360072.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.18.1.2/8.18.1.2) with ESMTP id 531NDik6030354; Wed, 2 Apr 2025 07:43:49 GMT DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ibm.com; h=cc :content-transfer-encoding:content-type:date:from:in-reply-to :message-id:mime-version:references:reply-to:subject:to; s=pp1; bh=exQLhMyg9qLptoM0qd7l3TQfUiHvZ6in9+giMllRWIQ=; b=tIArEPdSR4/p TOSU5exMpxX+iVuTzGk6OjWQ0mLySQwvrjveFS/pYs6wK/k5367vCcPIDS3Z1juS Xt8VVFzwIv/wTht7rYzX1Lq9kUQVq6W+C4asnrLuvE/AHZoXz8E81vevDgWyNf3h +/zO0q/lSL9BgTHkTraTJCQzjRlqooc6mbi20rNNztbU+updCzwOOBPNSO37lgMs oAphWmQPHORQoPri/8+e+H78iZI7s183269YnsG7cjYpzYLxxYrQ7t+qr1bvQyUd x5nE1dyJ5ez6ZBQcDjc1LWQoW/HHZRPmC+4JGeuY9zmBsrXvhfIyVHiFm9ZN9xpp n6exZC/7dA== Received: from pps.reinject (localhost [127.0.0.1]) by mx0a-001b2d01.pphosted.com (PPS) with ESMTPS id 45rmax3k4c-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Wed, 02 Apr 2025 07:43:49 +0000 (GMT) Received: from m0360072.ppops.net (m0360072.ppops.net [127.0.0.1]) by pps.reinject (8.18.0.8/8.18.0.8) with ESMTP id 5327aqFD029555; Wed, 2 Apr 2025 07:43:49 GMT Received: from ppma22.wdc07v.mail.ibm.com (5c.69.3da9.ip4.static.sl-reverse.com [169.61.105.92]) by mx0a-001b2d01.pphosted.com (PPS) with ESMTPS id 45rmax3k4a-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Wed, 02 Apr 2025 07:43:48 +0000 (GMT) Received: from pps.filterd (ppma22.wdc07v.mail.ibm.com [127.0.0.1]) by ppma22.wdc07v.mail.ibm.com (8.18.1.2/8.18.1.2) with ESMTP id 5323XGlI005210; Wed, 2 Apr 2025 07:43:48 GMT Received: from smtprelay02.fra02v.mail.ibm.com ([9.218.2.226]) by ppma22.wdc07v.mail.ibm.com (PPS) with ESMTPS id 45pujyxvca-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Wed, 02 Apr 2025 07:43:48 +0000 Received: from smtpav04.fra02v.mail.ibm.com (smtpav04.fra02v.mail.ibm.com [10.20.54.103]) by smtprelay02.fra02v.mail.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id 5327hicx28901764 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Wed, 2 Apr 2025 07:43:44 GMT Received: from smtpav04.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 4137E20043; Wed, 2 Apr 2025 07:43:44 +0000 (GMT) Received: from smtpav04.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 981B520040; Wed, 2 Apr 2025 07:43:41 +0000 (GMT) Received: from linux.ibm.com (unknown [9.126.150.29]) by smtpav04.fra02v.mail.ibm.com (Postfix) with ESMTP; Wed, 2 Apr 2025 07:43:41 +0000 (GMT) Date: Wed, 2 Apr 2025 13:13:40 +0530 From: Srikar Dronamraju To: Madadi Vineeth Reddy Cc: Madhavan Srinivasan , Michael Ellerman , Nicholas Piggin , Christophe Leroy , Naveen N Rao , Ard Biesheuvel , Eric Biggers , "Martin K . Petersen" , Andrew Morton , Yosry Ahmed , Tamir Duberstein , Shrikanth Hegde , linuxppc-dev@lists.ozlabs.org, LKML Subject: Re: [PATCH] powerpc/defconfigs: Set HZ=1000 on ppc64 and powernv defconfigs Message-ID: Reply-To: Srikar Dronamraju References: <20250330074734.16679-1-vineethr@linux.ibm.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20250330074734.16679-1-vineethr@linux.ibm.com> X-TM-AS-GCONF: 00 X-Proofpoint-GUID: hdZSBzQdGfFgFP7umnSi1s2PSOrMTdc_ X-Proofpoint-ORIG-GUID: luMiqcKl4tJtqmQ67hpumliGVgpaNFzL X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1095,Hydra:6.0.680,FMLib:17.12.68.34 definitions=2025-04-02_02,2025-04-01_01,2024-11-22_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 suspectscore=0 priorityscore=1501 clxscore=1011 lowpriorityscore=0 mlxscore=0 spamscore=0 adultscore=0 mlxlogscore=999 phishscore=0 impostorscore=0 bulkscore=0 malwarescore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.19.0-2502280000 definitions=main-2504020043 * Madadi Vineeth Reddy [2025-03-30 13:17:34]: > Commit 030bdc3fd080 ("powerpc/defconfigs: Set HZ=100 on pseries and ppc64 > defconfigs") lowered CONFIG_HZ from 250 to 100, citing reduced need for a > higher tick rate due to high-resolution timers and concerns about timer > interrupt overhead and cascading effects in the timer wheel. > > However, improvements have been made to the timer wheel algorithm since > then, particularly in eliminating cascading effects at the cost of minor > timekeeping inaccuracies. More details are available here > https://lwn.net/Articles/646950/. This removes the original concern about > cascading, and the reliance on high-resolution timers is not applicable > to the scheduler, which still depends on periodic ticks set by CONFIG_HZ. > > With the introduction of the EEVDF scheduler, users can specify custom > slices for workloads. The default base_slice is 3ms, but with CONFIG_HZ=100 > (10ms tick interval), base_slice is ineffective. Workloads like stress-ng > that do not voluntarily yield the CPU run for ~10ms before switching out. > Additionally, setting a custom slice below 3ms (e.g., 2ms) should lower > task latency, but this effect is lost due to the coarse 10ms tick. > > By increasing CONFIG_HZ to 1000 (1ms tick), base_slice is properly honored, > and user-defined slices work as expected. Benchmark results support this > change: > > Latency improvements in schbench with EEVDF under stress-ng-induced noise: > > Scheduler CONFIG_HZ Custom Slice 99th Percentile Latency (µs) > -------------------------------------------------------------------- > EEVDF 1000 No 0.30x > EEVDF 1000 2 ms 0.29x > EEVDF (default) 100 No 1.00x > > Switching to HZ=1000 reduces the 99th percentile latency in schbench by > ~70%. This improvement occurs because, with HZ=1000, stress-ng tasks run > for ~3ms before yielding, compared to ~10ms with HZ=100. As a result, > schbench gets CPU time sooner, reducing its latency. > > Daytrader Performance: > > Daytrader results show minor variation within standard deviation, > indicating no significant regression. > > Workload (Users/Instances) Throughput 1000HZ vs 100HZ (Std Dev%) > -------------------------------------------------------------------------- > 30 u, 1 i +3.01% (1.62%) > 60 u, 1 i +1.46% (2.69%) > 90 u, 1 i –1.33% (3.09%) > 30 u, 2 i -1.20% (1.71%) > 30 u, 3 i –0.07% (1.33%) > > Avg. Response Time: No Change (=) > > pgbench select queries: > > Metric 1000HZ vs 100HZ (Std Dev%) > ------------------------------------------------------------------ > Average TPS Change +2.16% (1.27%) > Average Latency Change –2.21% (1.21%) > > Average TPS: Higher the better > Average Latency: Lower the better > > pgbench shows both throughput and latency improvements beyond standard > deviation. > > Given these results and the improvements in timer wheel implementation, > increasing CONFIG_HZ to 1000 ensures that powerpc benefits from EEVDF’s > base_slice and allows fine-tuned scheduling for latency-sensitive > workloads. > > Signed-off-by: Madadi Vineeth Reddy Good work Vineeth, As pointed by you, the base slice is 3ms and having base slice as a multiple of tick will help. The numbers also support this change. Looks good to me. Reviewed-by: Srikar Dronamraju -- Thanks and Regards Srikar Dronamraju