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 11:49:36 +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 11:41, 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?

-- 
Pavel Begunkov