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 X-Spam-Level: X-Spam-Status: No, score=-13.2 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_IN_DEF_DKIM_WL autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9F4CEC48BE8 for ; Wed, 16 Jun 2021 19:07:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7F5CD610CA for ; Wed, 16 Jun 2021 19:07:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232329AbhFPTJV (ORCPT ); Wed, 16 Jun 2021 15:09:21 -0400 Received: from linux.microsoft.com ([13.77.154.182]:40004 "EHLO linux.microsoft.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232263AbhFPTJU (ORCPT ); Wed, 16 Jun 2021 15:09:20 -0400 Received: from mail-pf1-f182.google.com (mail-pf1-f182.google.com [209.85.210.182]) by linux.microsoft.com (Postfix) with ESMTPSA id D4FD820B6C50; Wed, 16 Jun 2021 12:07:13 -0700 (PDT) DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com D4FD820B6C50 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com; s=default; t=1623870433; bh=pMMmTT3PXQgTrevuxlj2F3JLlcd4EM9l6Fepj29X/Bg=; h=References:In-Reply-To:From:Date:Subject:To:Cc:From; b=et5fmwABi16nTifeqv5owKDbpO8Upojx4jogULjH35iG+VU2qzgIOkQSB+40B0esG Wam6aYGNXbuJ3uQm6BvTW69sFa4xi4qmwEIBwkn8G5wv6OmR+8OomBdF99AwSZvJc1 Uf510fCV9WWV72wjblSHguWSQzdzsPUtxfXw9eOo= Received: by mail-pf1-f182.google.com with SMTP id p13so3021834pfw.0; Wed, 16 Jun 2021 12:07:13 -0700 (PDT) X-Gm-Message-State: AOAM531oszfqMbna3A53N1neh5zAbBL8+6I7kkBR9p1OJ6K17L7M1EhD GirYLhPt57YmA1CjaYGTSvs0IwTzaAuAHMlQ1kI= X-Google-Smtp-Source: ABdhPJx9sD4Hv60I5h6KAYTe91Zo1s5HoDH8VwMli9/OBjeUKnpAHtllS55ebWXOgWb1A5g5i1Y5vWiMz6iFCmYlJ6k= X-Received: by 2002:a05:6a00:24d0:b029:2ed:c309:8b0f with SMTP id d16-20020a056a0024d0b02902edc3098b0fmr1281204pfv.41.1623870433450; Wed, 16 Jun 2021 12:07:13 -0700 (PDT) MIME-Version: 1.0 References: <20210615023812.50885-1-mcroce@linux.microsoft.com> <20210615023812.50885-2-mcroce@linux.microsoft.com> <6cff2a895db94e6fadd4ddffb8906a73@AcuMS.aculab.com> <1632006872b04c64be828fa0c4e4eae0@AcuMS.aculab.com> <20210616040132.7fbdf6fe@linux.microsoft.com> In-Reply-To: From: Matteo Croce Date: Wed, 16 Jun 2021 21:06:37 +0200 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH 1/3] riscv: optimized memcpy To: David Laight Cc: Bin Meng , Emil Renner Berthing , Gary Guo , "linux-riscv@lists.infradead.org" , "linux-kernel@vger.kernel.org" , "linux-arch@vger.kernel.org" , Paul Walmsley , Palmer Dabbelt , Albert Ou , Atish Patra , Akira Tsukamoto , Drew Fustini Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jun 16, 2021 at 10:24 AM David Laight wrote: > > From: Matteo Croce > > Sent: 16 June 2021 03:02 > ... > > > > That's a good idea, but if you read the replies to Gary's original > > > > patch > > > > https://lore.kernel.org/linux-riscv/20210216225555.4976-1-gary@garyguo.net/ > > > > .. both Gary, Palmer and David would rather like a C-based version. > > > > This is one attempt at providing that. > > > > > > Yep, I prefer C as well :) > > > > > > But if you check commit 04091d6, the assembly version was introduced > > > for KASAN. So if we are to change it back to C, please make sure KASAN > > > is not broken. > > > > ... > > Leaving out the first memcpy/set of every test which is always slower, (maybe > > because of a cache miss?), the current implementation copies 260 Mb/s when > > the low order bits match, and 114 otherwise. > > Memset is stable at 278 Mb/s. > > > > Gary's implementation is much faster, copies still 260 Mb/s when euqlly placed, > > and 230 Mb/s otherwise. Memset is the same as the current one. > > Any idea what the attainable performance is for the cpu you are using? > Since both memset and memcpy are running at much the same speed > I suspect it is all limited by the writes. > > 272MB/s is only 34M writes/sec. > This seems horribly slow for a modern cpu. > So is this actually really limited by the cache writes to physical memory? > > You might want to do some tests (userspace is fine) where you > check much smaller lengths that definitely sit within the data cache. > I get similar results in userspace, this tool write to RAM with variable data width: root@beaglev:~/src# ./unalign_check 1 0 1 size: 1 Mb write size: 8 bit unalignment: 0 byte elapsed time: 0.01 sec throughput: 124.36 Mb/s # ./unalign_check 1 0 8 size: 1 Mb write size: 64 bit unalignment: 0 byte elapsed time: 0.00 sec throughput: 252.12 Mb/s > It is also worth checking how much overhead there is for > short copies - they are almost certainly more common than > you might expect. > This is one problem with excessive loop unrolling - the 'special > cases' for the ends of the buffer start having a big effect > on small copies. > I too believe that they are much more common than long ones. Indeed, I wish to reduce the MIN_THRESHOLD value from 64 to 32 or even 16. Or having it dependend on the word size, e.g. sizeof(long) * 2. Suggestions? > For cpu that support misaligned memory accesses, one 'trick' > for transfers longer than a 'word' is to do a (probably) misaligned > transfer of the last word of the buffer first followed by the > transfer of the rest of the buffer (overlapping a few bytes at the end). > This saves on conditionals and temporary values. > > David > > - > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK > Registration No: 1397386 (Wales) > Regards, -- per aspera ad upstream