From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from aserp2120.oracle.com (aserp2120.oracle.com [141.146.126.78]) by sourceware.org (Postfix) with ESMTPS id 7FFF23857C5A for ; Wed, 23 Sep 2020 20:09:49 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 7FFF23857C5A Received: from pps.filterd (aserp2120.oracle.com [127.0.0.1]) by aserp2120.oracle.com (8.16.0.42/8.16.0.42) with SMTP id 08NK9DUY166442 for ; Wed, 23 Sep 2020 20:09:48 GMT Received: from userp3030.oracle.com (userp3030.oracle.com [156.151.31.80]) by aserp2120.oracle.com with ESMTP id 33q5rgjy2b-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL) for ; Wed, 23 Sep 2020 20:09:48 +0000 Received: from pps.filterd (userp3030.oracle.com [127.0.0.1]) by userp3030.oracle.com (8.16.0.42/8.16.0.42) with SMTP id 08NK5Qaq030365 for ; Wed, 23 Sep 2020 20:09:47 GMT Received: from userv0121.oracle.com (userv0121.oracle.com [156.151.31.72]) by userp3030.oracle.com with ESMTP id 33nux1nau1-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK) for ; Wed, 23 Sep 2020 20:09:47 +0000 Received: from abhmp0002.oracle.com (abhmp0002.oracle.com [141.146.116.8]) by userv0121.oracle.com (8.14.4/8.13.8) with ESMTP id 08NK9kE4032705 for ; Wed, 23 Sep 2020 20:09:46 GMT Received: from arm34vm.us.oracle.com (/10.211.14.34) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Wed, 23 Sep 2020 13:09:46 -0700 From: Patrick McGehearty To: libc-alpha@sourceware.org Subject: [PATCH v2] Reversing calculation of __x86_shared_non_temporal_threshold Date: Wed, 23 Sep 2020 20:09:41 +0000 Message-Id: <1600891781-9272-1-git-send-email-patrick.mcgehearty@oracle.com> X-Mailer: git-send-email 1.8.3.1 X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9753 signatures=668680 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 malwarescore=0 mlxscore=0 adultscore=0 bulkscore=0 mlxlogscore=999 phishscore=0 suspectscore=0 spamscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2006250000 definitions=main-2009230150 X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9753 signatures=668680 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 spamscore=0 impostorscore=0 clxscore=1015 suspectscore=0 phishscore=0 malwarescore=0 priorityscore=1501 mlxlogscore=999 adultscore=0 bulkscore=0 mlxscore=0 lowpriorityscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2006250000 definitions=main-2009230151 X-Spam-Status: No, score=-12.4 required=5.0 tests=BAYES_00, DKIMWL_WL_HIGH, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, GIT_PATCH_0, RCVD_IN_MSPIKE_H2, SPF_HELO_PASS, SPF_PASS, TXREP, UNPARSEABLE_RELAY autolearn=ham autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: libc-alpha@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 23 Sep 2020 20:09:51 -0000 The __x86_shared_non_temporal_threshold determines when memcpy on x86 uses non_temporal stores to avoid pushing other data out of the last level cache. This patch proposes to revert the calculation change made by H.J. Lu's patch of June 2, 2017. H.J. Lu's patch selected a threshold suitable for a single thread getting maximum performance. It was tuned using the single threaded large memcpy micro benchmark on an 8 core processor. The last change changes the threshold from using 3/4 of one thread's share of the cache to using 3/4 of the entire cache of a multi-threaded system before switching to non-temporal stores. Multi-threaded systems with more than a few threads are server-class and typically have many active threads. If one thread consumes 3/4 of the available cache for all threads, it will cause other active threads to have data removed from the cache. Two examples show the range of the effect. John McCalpin's widely parallel Stream benchmark, which runs in parallel and fetches data sequentially, saw a 20% slowdown with this patch on an internal system test of 128 threads. This regression was discovered when comparing OL8 performance to OL7. An example that compares normal stores to non-temporal stores may be found at https://vgatherps.github.io/2018-09-02-nontemporal/. A simple test shows performance loss of 400 to 500% due to a failure to use nontemporal stores. These performance losses are most likely to occur when the system load is heaviest and good performance is critical. The tunable x86_non_temporal_threshold can be used to override the default for the knowledgable user who really wants maximum cache allocation to a single thread in a multi-threaded system. The manual entry for the tunable has been expanded to provide more information about its purpose. modified: sysdeps/x86/cacheinfo.c modified: manual/tunables.texi --- manual/tunables.texi | 6 +++++- sysdeps/x86/cacheinfo.c | 12 +++++++----- 2 files changed, 12 insertions(+), 6 deletions(-) diff --git a/manual/tunables.texi b/manual/tunables.texi index b6bb54d..94d4fbd 100644 --- a/manual/tunables.texi +++ b/manual/tunables.texi @@ -364,7 +364,11 @@ set shared cache size in bytes for use in memory and string routines. @deftp Tunable glibc.tune.x86_non_temporal_threshold The @code{glibc.tune.x86_non_temporal_threshold} tunable allows the user -to set threshold in bytes for non temporal store. +to set threshold in bytes for non temporal store. Non temporal stores +give a hint to the hardware to move data directly to memory without +displacing other data from the cache. This tunable is used by some +platforms to determine when to use non temporal stores in operations +like memmove and memcpy. This tunable is specific to i386 and x86-64. @end deftp diff --git a/sysdeps/x86/cacheinfo.c b/sysdeps/x86/cacheinfo.c index b9444dd..c6767d9 100644 --- a/sysdeps/x86/cacheinfo.c +++ b/sysdeps/x86/cacheinfo.c @@ -778,14 +778,16 @@ intel_bug_no_cache_info: __x86_shared_cache_size = shared; } - /* The large memcpy micro benchmark in glibc shows that 6 times of - shared cache size is the approximate value above which non-temporal - store becomes faster on a 8-core processor. This is the 3/4 of the - total shared cache size. */ + /* The default setting for the non_temporal threshold is 3/4 + of one thread's share of the chip's cache. While higher + single thread performance may be observed with a higher + threshold, having a single thread use more than it's share + of the cache will negatively impact the performance of + other threads running on the chip. */ __x86_shared_non_temporal_threshold = (cpu_features->non_temporal_threshold != 0 ? cpu_features->non_temporal_threshold - : __x86_shared_cache_size * threads * 3 / 4); + : __x86_shared_cache_size * 3 / 4); } #endif -- 1.8.3.1