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Thu, 17 Jun 2021 22:30:06 +0100 Received: from AcuMS.Aculab.com ([fe80::994c:f5c2:35d6:9b65]) by AcuMS.aculab.com ([fe80::994c:f5c2:35d6:9b65%12]) with mapi id 15.00.1497.018; Thu, 17 Jun 2021 22:30:06 +0100 From: David Laight To: 'Matteo Croce' , Guo Ren CC: linux-riscv , Linux Kernel Mailing List , linux-arch , Paul Walmsley , Palmer Dabbelt , Albert Ou , Atish Patra , Emil Renner Berthing , "Akira Tsukamoto" , Drew Fustini , Bin Meng Subject: RE: [PATCH 1/3] riscv: optimized memcpy Thread-Topic: [PATCH 1/3] riscv: optimized memcpy Thread-Index: AQHXYuC4XkdMIImxVUmoQbZ37iIZIqsYtSAg Date: Thu, 17 Jun 2021 21:30:06 +0000 Message-ID: References: <20210615023812.50885-1-mcroce@linux.microsoft.com> <20210615023812.50885-2-mcroce@linux.microsoft.com> In-Reply-To: Accept-Language: en-GB, en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=C51A453 smtp.mailfrom=david.laight@aculab.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Language: en-US X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210617_143014_982588_CF8862D0 X-CRM114-Status: GOOD ( 20.06 ) X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org From: Matteo Croce > Sent: 16 June 2021 19:52 > To: Guo Ren > > On Wed, Jun 16, 2021 at 1:46 PM Guo Ren wrote: > > > > Hi Matteo, > > > > Have you tried Glibc generic implementation code? > > ref: https://lore.kernel.org/linux-arch/20190629053641.3iBfk9- > I_D29cDp9yJnIdIg7oMtHNZlDmhLQPTumhEc@z/#t > > > > If Glibc codes have the same performance in your hardware, then you > > could give a generic implementation first. Isn't that a byte copy loop - the performance of that ought to be terrible. ... > I had a look, it seems that it's a C unrolled version with the > 'register' keyword. > The same one was already merged in nios2: > https://elixir.bootlin.com/linux/latest/source/arch/nios2/lib/memcpy.c#L68 I know a lot about the nios2 instruction timings. (I've looked at code execution in the fpga's intel 'logic analiser.) It is a very simple 4-clock pipeline cpu with a 2-clock delay before a value read from 'tightly coupled memory' (aka cache) can be used in another instruction. There is also a subtle pipeline stall if a read follows a write to the same memory block because the write is executed one clock later - and would collide with the read. Since it only ever executes one instruction per clock loop unrolling does help - since you never get the loop control 'for free'. OTOH you don't need to use that many registers. But an unrolled loop should approach 2 bytes/clock (32bit cpu). > I copied _wordcopy_fwd_aligned() from Glibc, and I have a very similar > result of the other versions: > > [ 563.359126] Strings selftest: memcpy(src+7, dst+7): 257 Mb/s What clock speed is that running at? It seems very slow for a 64bit cpu (that isn't an fpga soft-cpu). While the small riscv cpu might be similar to the nios2 (and mips for that matter), there are also bigger/faster cpu. I'm sure these can execute multiple instructions/clock and possible even read and write at the same time. Unless they also support significant instruction re-ordering the trivial copy loops are going to be slow on such cpu. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales) _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv