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Re: DMA is obsolete

From Lars Poulsen <lars@cleo.beagle-ears.com>
Newsgroups comp.arch
Subject Re: DMA is obsolete
Date 2025-04-26 16:28 +0000
Organization A noiseless patient Spider
Message-ID <slrn100q2dv.eisl.lars@cleo.beagle-ears.com> (permalink)
References <vuj131$fnu$1@gal.iecc.com>

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On 2025-04-26, John Levine <johnl@taugh.com> wrote:
> Well, not entirely.  This preprint argues that in environments with
> lots of cores and where latency is an issue, programmed I/O can outperform
> DMA.
>
> Rethinking Programmed I/O for Fast Devices, Cheap Cores, and Coherent Interconnects
>
> Anastasiia Ruzhanskaia, Pengcheng Xu, David Cock, Timothy Roscoe
>
> Conventional wisdom holds that an efficient interface between an OS
> running on a CPU and a high-bandwidth I/O device should use Direct
> Memory Access (DMA) to offload data transfer, descriptor rings for
> buffering and queuing, and interrupts for asynchrony between cores and
> device. In this paper we question this wisdom in the light of two
> trends: modern and emerging cache-coherent interconnects like CXL3.0,
> and workloads, particularly microservices and serverless computing.
> Like some others before us, we argue that the assumptions of the
> DMA-based model are obsolete, and in many use-cases programmed I/O,
> where the CPU explicitly transfers data and control information to and
> from a device via loads and stores, delivers a more efficient system.
> However, we push this idea much further. We show, in a real hardware
> implementation, the gains in latency for fine-grained communication
> achievable using an open cache-coherence protocol which exposes cache
> transitions to a smart device, and that throughput is competitive with
> DMA over modern interconnects. We also demonstrate three use-cases:
> fine-grained RPC-style invocation of functions on an accelerator,
> offloading of operators in a streaming dataflow engine, and a network
> interface targeting serverless functions, comparing our use of
> coherence with both traditional DMA-style interaction and a
> highly-optimized implementation using memory-mapped programmed I/O
> over PCIe.
>
> https://arxiv.org/abs/2409.08141

What is the difference between DMA and message-passing to another core
doing CMOV loop at the ISA level?

DMA means doing that it the micro-engine instead of at the ISA level.
Same difference.

What am I missing?

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Thread

DMA is obsolete John Levine <johnl@taugh.com> - 2025-04-26 16:19 +0000
  Re: DMA is obsolete Lars Poulsen <lars@cleo.beagle-ears.com> - 2025-04-26 16:28 +0000
    Re: DMA is obsolete Terje Mathisen <terje.mathisen@tmsw.no> - 2025-04-26 19:28 +0200
    Re: DMA is obsolete Theo <theom+news@chiark.greenend.org.uk> - 2025-04-27 19:35 +0100
      Re: DMA is obsolete mitchalsup@aol.com (MitchAlsup1) - 2025-04-27 20:49 +0000
        Re: DMA is obsolete scott@slp53.sl.home (Scott Lurndal) - 2025-04-27 22:37 +0000
      Re: DMA is obsolete Lawrence D'Oliveiro <ldo@nz.invalid> - 2025-04-28 01:20 +0000
  Re: DMA is obsolete scott@slp53.sl.home (Scott Lurndal) - 2025-04-26 17:29 +0000
    Re: DMA is obsolete mitchalsup@aol.com (MitchAlsup1) - 2025-04-26 19:25 +0000
      Re: DMA is obsolete scott@slp53.sl.home (Scott Lurndal) - 2025-04-27 14:01 +0000
        Re: DMA is obsolete scott@slp53.sl.home (Scott Lurndal) - 2025-04-27 16:12 +0000
      Re: DMA is obsolete scott@slp53.sl.home (Scott Lurndal) - 2025-04-27 14:02 +0000
      Re: DMA is obsolete Theo <theom+news@chiark.greenend.org.uk> - 2025-04-27 20:13 +0100
        Re: DMA is obsolete mitchalsup@aol.com (MitchAlsup1) - 2025-04-27 20:45 +0000
          Re: DMA is obsolete scott@slp53.sl.home (Scott Lurndal) - 2025-04-27 22:44 +0000
      Re: DMA is obsolete cross@spitfire.i.gajendra.net (Dan Cross) - 2025-05-01 13:07 +0000
        Re: DMA is obsolete mitchalsup@aol.com (MitchAlsup1) - 2025-05-01 22:03 +0000
          Re: DMA is obsolete cross@spitfire.i.gajendra.net (Dan Cross) - 2025-05-02 02:15 +0000
            Re: DMA is obsolete anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2025-05-02 05:34 +0000
              Re: DMA is obsolete cross@spitfire.i.gajendra.net (Dan Cross) - 2025-05-02 15:02 +0000
                Re: DMA is obsolete anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2025-05-03 06:11 +0000
                Re: DMA is obsolete Robert Finch <robfi680@gmail.com> - 2025-05-03 06:32 -0400
                Re: DMA is obsolete cross@spitfire.i.gajendra.net (Dan Cross) - 2025-05-03 13:33 +0000
                IP (was: DMA is obsolete) Stefan Monnier <monnier@iro.umontreal.ca> - 2025-05-03 10:50 -0400
                Re: IP (was: DMA is obsolete) Thomas Koenig <tkoenig@netcologne.de> - 2025-05-03 15:15 +0000
                Re: IP (was: DMA is obsolete) John Levine <johnl@taugh.com> - 2025-05-03 15:46 +0000
                Re: IP (was: DMA is obsolete) cross@spitfire.i.gajendra.net (Dan Cross) - 2025-05-03 16:52 +0000
                Re: IP (was: DMA is obsolete) scott@slp53.sl.home (Scott Lurndal) - 2025-05-03 21:31 +0000
                Re: IP Stefan Monnier <monnier@iro.umontreal.ca> - 2025-05-03 23:04 -0400
                Re: IP cross@spitfire.i.gajendra.net (Dan Cross) - 2025-05-04 09:56 +0000
                Re: IP Thomas Koenig <tkoenig@netcologne.de> - 2025-05-04 10:17 +0000
                Re: IP mitchalsup@aol.com (MitchAlsup1) - 2025-05-04 18:16 +0000
                Re: IP Bill Findlay <findlaybill@blueyonder.co.uk> - 2025-05-04 19:37 +0100
                Re: IP Lawrence D'Oliveiro <ldo@nz.invalid> - 2025-05-04 21:31 +0000
                Re: DMA is obsolete Lawrence D'Oliveiro <ldo@nz.invalid> - 2025-05-04 06:44 +0000
                Re: DMA is obsolete scott@slp53.sl.home (Scott Lurndal) - 2025-05-03 21:53 +0000
                Re: DMA is obsolete mitchalsup@aol.com (MitchAlsup1) - 2025-05-03 23:02 +0000
                Re: DMA is obsolete cross@spitfire.i.gajendra.net (Dan Cross) - 2025-05-21 12:36 +0000
            Re: DMA is obsolete mitchalsup@aol.com (MitchAlsup1) - 2025-05-02 17:40 +0000
              Re: DMA is obsolete Terje Mathisen <terje.mathisen@tmsw.no> - 2025-05-03 14:29 +0200
                ND-10 (was Re: DMA is obsolete) Lars Poulsen <lars@beagle-ears.com> - 2025-05-03 23:30 +0000
                Re: ND-10 (was Re: DMA is obsolete) Lawrence D'Oliveiro <ldo@nz.invalid> - 2025-05-04 02:10 +0000

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