Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail
From: Stefan Monnier <monnier@iro.umontreal.ca>
Newsgroups: comp.arch
Subject: Re: rep movsb vs. simpler instructions for memcpy/memmove
Date: Fri, 14 Mar 2025 16:37:08 -0400
Organization: A noiseless patient Spider
Lines: 13
Message-ID: <jwv4izvz742.fsf-monnier+comp.arch@gnu.org>
References: <vpufbv$4qc5$1@dont-email.me> <20250312140915.000010a8@yahoo.com>
	<2025Mar12.174636@mips.complang.tuwien.ac.at>
	<a296144c60c9774898235f505bc4c370@www.novabbs.org>
	<jwvy0x93vb5.fsf-monnier+comp.arch@gnu.org>
	<61cab9791f342672dcbd5dfd539cc5cc@www.novabbs.org>
	<jwv7c4s3n4d.fsf-monnier+comp.arch@gnu.org>
	<3104c2ab707086659d698a0377450527@www.novabbs.org>
	<20250313225516.00004206@yahoo.com> <5jIAP.61128$Xq5f.38323@fx38.iad>
	<20250314001619.000004fa@yahoo.com>
	<2025Mar14.141837@mips.complang.tuwien.ac.at>
MIME-Version: 1.0
Content-Type: text/plain
Injection-Date: Fri, 14 Mar 2025 21:37:09 +0100 (CET)
Injection-Info: dont-email.me; posting-host="039f5f6059b01691c21f036354f8b6f2";
	logging-data="2138733"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1+UYxInqrFYRaQzXevuvAat5tHvBqlyNy8="
User-Agent: Gnus/5.13 (Gnus v5.13)
Cancel-Lock: sha1:MDI9CiJDohbfSwwGVbmL36lEa20=
	sha1:iqYqA7O6GNHDCCku6PuFM/3UCTU=
Bytes: 2057

Anton Ertl [2025-03-14 13:18:37] wrote:
> The usual case where "from" is memory-mapped I/O and "to" is
> cache-coherent is when loading from an NVME SSD.  AFAIK this is
> usually done in larger block sizes, because of the overhead of setting
> up the DMA, and is usually done in an asynchronous way.

Also, is the data transfer separate from the "disk" access?  I'd expect
that the NVMe interface lets the CPU say "read block B and DMA it to
DRAM at address X" (after which we get an interrupt), so there is no
opportunity for a `rep movsb` or `MM` instruction to do part of the job.


        Stefan