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From: Jens Axboe <[email protected]>
To: Andy Lutomirski <[email protected]>
Cc: Linus Torvalds <[email protected]>,
	Dave Chinner <[email protected]>,
	Matthew Wilcox <[email protected]>,
	Stefan Metzmacher <[email protected]>,
	linux-fsdevel <[email protected]>,
	Linux API Mailing List <[email protected]>,
	io-uring <[email protected]>,
	"[email protected]" <[email protected]>,
	Al Viro <[email protected]>,
	Samba Technical <[email protected]>
Subject: Re: copy on write for splice() from file to pipe?
Date: Fri, 10 Feb 2023 14:51:52 -0700	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>

On 2/10/23 2:27 PM, Jens Axboe wrote:
> On 2/10/23 2:14?PM, Andy Lutomirski wrote:
>> On Fri, Feb 10, 2023 at 12:50 PM Jens Axboe <[email protected]> wrote:
>>>
>>> On 2/10/23 1:44?PM, Linus Torvalds wrote:
>>>> On Fri, Feb 10, 2023 at 12:39 PM Jens Axboe <[email protected]> wrote:
>>>>>
>>>>> Right, I'm referencing doing zerocopy data sends with io_uring, using
>>>>> IORING_OP_SEND_ZC. This isn't from a file, it's from a memory location,
>>>>> but the important bit here is the split notifications and how you
>>>>> could wire up a OP_SENDFILE similarly to what Andy described.
>>>>
>>>> Sure, I think it's much more reasonable with io_uring than with splice itself.
>>>>
>>>> So I was mainly just reacting to the "strict-splice" thing where Andy
>>>> was talking about tracking the page refcounts. I don't think anything
>>>> like that can be done at a splice() level, but higher levels that
>>>> actually know about the whole IO might be able to do something like
>>>> that.
>>>>
>>>> Maybe we're just talking past each other.
>>>
>>> Maybe slightly, as I was not really intending to comment on the strict
>>> splice thing. But yeah I agree on splice, it would not be trivial to do
>>> there. At least with io_uring we have the communication channel we need.
>>> And tracking page refcounts seems iffy and fraught with potential
>>> issues.
>>>
>>
>> Hmm.
>>
>> Are there any real-world use cases for zero-copy splice() that
>> actually depend on splicing from a file to a pipe and then later from
>> the pipe to a socket (or file or whatever)?  Or would everything
>> important be covered by a potential new io_uring operation that copies
>> from one fd directly to another fd?
> 
> I think it makes sense. As Linus has referenced, the sex appeal of
> splice is the fact that it is dealing with pipes, and you can access
> these internal buffers through other means. But that is probably largely
> just something that is sexy design wise, nothing that _really_ matters
> in practice. And the pipes do get in the way, for example I had to add
> pipe resizing fcntl helpers to bump the size. If you're doing a plain
> sendfile, the pipes just kind of get in the way too imho.
> 
> Another upside (from the io_uring) perspective is that splice isn't very
> efficient through io_uring, as it requires offload to io-wq. This could
> obviously be solved by some refactoring in terms of non-blocking, but it
> hasn't really been that relevant (and nobody has complained about it). A
> new sendfile op would nicely get around that too as it could be designed
> with async in nature, rather than the classic sync syscall model that
> splice follows.

Speaking of splice/io_uring, Ming posted this today:

https://lore.kernel.org/io-uring/[email protected]/

-- 
Jens Axboe



  reply	other threads:[~2023-02-10 21:52 UTC|newest]

Thread overview: 64+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-02-09 13:55 copy on write for splice() from file to pipe? Stefan Metzmacher
2023-02-09 14:11 ` Matthew Wilcox
2023-02-09 14:29   ` Stefan Metzmacher
2023-02-09 16:41 ` Linus Torvalds
2023-02-09 19:17   ` Stefan Metzmacher
2023-02-09 19:36     ` Linus Torvalds
2023-02-09 19:48       ` Linus Torvalds
2023-02-09 20:33         ` Jeremy Allison
2023-02-10 20:45         ` Stefan Metzmacher
2023-02-10 20:51           ` Linus Torvalds
2023-02-10  2:16   ` Dave Chinner
2023-02-10  4:06     ` Dave Chinner
2023-02-10  4:44       ` Matthew Wilcox
2023-02-10  6:57         ` Dave Chinner
2023-02-10 15:14           ` Andy Lutomirski
2023-02-10 16:33             ` Linus Torvalds
2023-02-10 17:57               ` Andy Lutomirski
2023-02-10 18:19                 ` Jeremy Allison
2023-02-10 19:29                   ` Stefan Metzmacher
2023-02-10 18:37                 ` Linus Torvalds
2023-02-10 19:01                   ` Andy Lutomirski
2023-02-10 19:18                     ` Linus Torvalds
2023-02-10 19:27                       ` Jeremy Allison
2023-02-10 19:42                         ` Stefan Metzmacher
2023-02-10 19:42                         ` Linus Torvalds
2023-02-10 19:54                           ` Stefan Metzmacher
2023-02-10 19:29                       ` Linus Torvalds
2023-02-13  9:07                         ` Herbert Xu
2023-02-10 19:55                       ` Andy Lutomirski
2023-02-10 20:27                         ` Linus Torvalds
2023-02-10 20:32                           ` Jens Axboe
2023-02-10 20:36                             ` Linus Torvalds
2023-02-10 20:39                               ` Jens Axboe
2023-02-10 20:44                                 ` Linus Torvalds
2023-02-10 20:50                                   ` Jens Axboe
2023-02-10 21:14                                     ` Andy Lutomirski
2023-02-10 21:27                                       ` Jens Axboe
2023-02-10 21:51                                         ` Jens Axboe [this message]
2023-02-10 22:08                                           ` Linus Torvalds
2023-02-10 22:16                                             ` Jens Axboe
2023-02-10 22:17                                             ` Linus Torvalds
2023-02-10 22:25                                               ` Jens Axboe
2023-02-10 22:35                                                 ` Linus Torvalds
2023-02-10 22:51                                                   ` Jens Axboe
2023-02-11  3:18                                             ` Ming Lei
2023-02-11  6:17                                               ` Ming Lei
2023-02-11 14:13                                               ` Jens Axboe
2023-02-11 15:05                                                 ` Ming Lei
2023-02-11 15:33                                                   ` Jens Axboe
2023-02-11 18:57                                                     ` Linus Torvalds
2023-02-12  2:46                                                       ` Jens Axboe
2023-02-10  4:47       ` Linus Torvalds
2023-02-10  6:19         ` Dave Chinner
2023-02-10 17:23           ` Linus Torvalds
2023-02-10 17:47             ` Linus Torvalds
2023-02-13  9:28               ` Herbert Xu
2023-02-10 22:41             ` David Laight
2023-02-10 22:51               ` Jens Axboe
2023-02-13  9:30               ` Herbert Xu
2023-02-13  9:25           ` Herbert Xu
2023-02-13 18:01             ` Andy Lutomirski
2023-02-14  1:22               ` Herbert Xu
2023-02-17 23:13                 ` Andy Lutomirski
2023-02-20  4:54                   ` Herbert Xu

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