Re: [RFC, PATCH] Extensible AIO interface

[Jeff Moyer]

Kent Overstreet <koverstreet@xxxxxxxxxx> writes:

> On Tue, Oct 02, 2012 at 01:41:17PM -0400, Jeff Moyer wrote:
>> Kent Overstreet <koverstreet@xxxxxxxxxx> writes:
>> 
>> > So, I and other people keep running into things where we really need to
>> > add an interface to pass some auxiliary... stuff along with a pread() or
>> > pwrite().
>> >
>> > A few examples:
>> >
>> > * IO scheduler hints. Some userspace program wants to, per IO, specify
>> > either priorities or a cgroup - by specifying a cgroup you can have a
>> > fileserver in userspace that makes use of cfq's per cgroup bandwidth
>> > quotas.
>> 
>> You can do this today by splitting I/O between processes and placing
>> those processes in different cgroups.  For io priority, there is
>> ioprio_set, which incurs an extra system call, but can be used.  Not
>> elegant, but possible.
>
> Yes - those are things I'm trying to replace. Doing it that way is a
> real pain, both as it's a lousy interface for this and it does impact
> performance (ioprio_set doesn't really work too well with aio, too).

ioprio_set works fine with aio, since the I/O is issued in the caller's
context.  Perhaps you're thinking of writeback I/O?

>> > * Cache hints. For bcache and other things, userspace may want to specify
>> > "this data should be cached", "this data should bypass the cache", etc.
>> 
>> Please explain how you will differentiate this from posix_fadvise.
>
> Oh sorry, I think about SSD caching so much I forget to say that's what
> I'm talking about. posix_fadvise is for the page cache, we want
> something different for an SSD cache (IMO it'd be really ugly to use it
> for both, and posix_fadvise() can't really specifify everything we'd
> want to for an SSD cache).

DESCRIPTION
       Programs can use posix_fadvise() to announce an intention to
       access file data in a specific pattern in the future, thus
       allowing the kernel to perform appropriate optimizations.

That description seems broad enough to include disk caches as well.  You
haven't exactly stated what's missing.

>> > Hence, AIO attributes.
>> 
>> *No.*  Start with the non-AIO case first.
>
> Why? It is orthogonal to AIO (and I should make that clearer), but to do
> it for sync IO we'd need new syscalls that take an extra argument so IMO
> it's a bit easier to start with AIO.
>
> Might be worth implementing the sync interface sooner rather than later
> just to discover any potential issues, I suppose.

Looking back to preadv and pwritev, it was wrong to only add them to
libaio (and that later got corrected).  I'd just like to see things
start out with the sync interfaces, since you'll get more eyes on the
code (not everyone cares about aio) and that helps to work out any
interface issues.

>> > * FUTURE STUFF:
>> >
>> > Return values:
>> >
>> > Some attributes are probably going to want to return something to
>> > userspace.
>> >
>> > If nothing else, we want this so that userspace can tell if anything
>> > handled the attributes it specified - as dynamic as the io stack can be,
>> > with something extensible like this there really isn't any generic way
>> > of knowing ahead of time if something is going to interpret any
>> > attribute - we want to return at least an error code.
>> 
>> Seems odd to me.  Why not expose supported attributes via some other
>> call?  fcntl?
>
> It's not possible in general - consider stacking block devices, and
> attrs that are supported only by specific block drivers. I.e. if you've
> got lvm on top of bcache or bcache on top of md, we can pass the attr
> down with the IO but we can't determine ahead of time, in general, where
> the IO is going to go.

If the io stack is static (meaning you setup a device once, then open it
and do io to it, and it doesn't change while you're doing io), how would
you not know where the IO is going to go?

> But that probably isn't true for most attrs so it probably would be a
> good idea to have an interface for querying what's supported, and even
> for device specific ones you could query what a device supports.

OK.

>> > One could imagine sticking the return in the attribute itself, but I
>> > don't want to do this. For some things (checksums), the attribute will
>> > contain a pointer to a buffer - that's fine. But I don't want the
>> > attributes themselves to be writeable.
>> 
>> One could imagine that attributes don't return anything, because, well,
>> they're properties of something else, and properties don't return
>> anything.
>
> With a strict definition of attribute, yeah. One of the real uses cases
> we have for this is per IO timings, for aio - right now we've got an
> interface for the kernel to tell userspace how long a syscall took
> (don't think it's upstream yet - Paul's been behind that stuff), but it
> only really makes sense with synchronous syscalls.

Something beyond recording the time spent in the kernel?  Paul who?  I
agree the per io timing for aio may be coarse-grained today (you can
time the difference between io_submit returning and the event being
returned by io_getevents, but that says nothing of when the io was
issued to the block layer).  I'm curious to know exactly what
granularity you want here, and what an application would do with that
information.  You can currently access a whole lot of detail of the io
path through blktrace, but that is not easily done from within an
application.

> These AIO attributes would be useful for that too, but I'd _much_ prefer
> if the timing information was explicitly returned instead of using a
> pointer to a buffer.

I'm having a hard time understanding exactly what you are timing.

Cheers,
Jeff
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