linux-kernel - Re: [PATCH 0/3] readfile(2): a new syscall to make open/read/close faster

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Date:   Wed, 15 Jul 2020 12:00:19 +0300
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Pavel Machek <pavel@...x.de>, Greg KH <gregkh@...uxfoundation.org>,
        Jan Ziak <0xe2.0x9a.0x9b@...il.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        linux-man <linux-man@...r.kernel.org>,
        Michael Kerrisk <mtk.manpages@...il.com>, shuah@...nel.org,
        Al Viro <viro@...iv.linux.org.uk>, io-uring@...r.kernel.org
Subject: Re: [PATCH 0/3] readfile(2): a new syscall to make open/read/close
 faster

On 15/07/2020 11:49, Pavel Begunkov wrote:
> On 15/07/2020 11:41, Miklos Szeredi wrote:
>> On Wed, Jul 15, 2020 at 10:33 AM Pavel Begunkov <asml.silence@...il.com> wrote:
>>>
>>> On 14/07/2020 14:55, Miklos Szeredi wrote:
>>>> On Tue, Jul 14, 2020 at 1:36 PM Pavel Begunkov <asml.silence@...il.com> wrote:
>>>>>
>>>>> On 14/07/2020 11:07, Miklos Szeredi wrote:
>>>>>> On Tue, Jul 14, 2020 at 8:51 AM Pavel Machek <pavel@...x.de> wrote:
>>>>>>>
>>>>>>> Hi!
>>>>>>>
>>>>>>>>> At first, I thought that the proposed system call is capable of
>>>>>>>>> reading *multiple* small files using a single system call - which
>>>>>>>>> would help increase HDD/SSD queue utilization and increase IOPS (I/O
>>>>>>>>> operations per second) - but that isn't the case and the proposed
>>>>>>>>> system call can read just a single file.
>>>>>>>>
>>>>>>>> If you want to do this for multple files, use io_ring, that's what it
>>>>>>>> was designed for.  I think Jens was going to be adding support for the
>>>>>>>> open/read/close pattern to it as well, after some other more pressing
>>>>>>>> features/fixes were finished.
>>>>>>>
>>>>>>> What about... just using io_uring for single file, too? I'm pretty
>>>>>>> sure it can be wrapped in a library that is simple to use, avoiding
>>>>>>> need for new syscall.
>>>>>>
>>>>>> Just wondering:  is there a plan to add strace support to io_uring?
>>>>>> And I don't just mean the syscalls associated with io_uring, but
>>>>>> tracing the ring itself.
>>>>>
>>>>> What kind of support do you mean? io_uring is asynchronous in nature
>>>>> with all intrinsic tracing/debugging/etc. problems of such APIs.
>>>>> And there are a lot of handy trace points, are those not enough?
>>>>>
>>>>> Though, this can be an interesting project to rethink how async
>>>>> APIs are worked with.
>>>>
>>>> Yeah, it's an interesting problem.  The uring has the same events, as
>>>> far as I understand, that are recorded in a multithreaded strace
>>>> output (syscall entry, syscall exit); nothing more is needed>
>>>> I do think this needs to be integrated into strace(1), otherwise the
>>>> usefulness of that tool (which I think is *very* high) would go down
>>>> drastically as io_uring usage goes up.
>>>
>>> Not touching the topic of usefulness of strace + io_uring, but I'd rather
>>> have a tool that solves a problem, than a problem that created and honed
>>> for a tool.
>>
>> Sorry, I'm not getting the metaphor.  Can you please elaborate?
> 
> Sure, I mean _if_ there are tools that conceptually suit better, I'd
> prefer to work with them, then trying to shove a new and possibly alien
> infrastructure into strace.
> 
> But my knowledge of strace is very limited, so can't tell whether that's
> the case. E.g. can it utilise static trace points?

I think, if you're going to push this idea, we should start a new thread
CC'ing strace devs.
	
-- 
Pavel Begunkov