Re: sleeps and waits during io_submit

[Avi Kivity <avi@scylladb.com>]

On 12/01/2015 11:04 PM, Dave Chinner wrote:
> On Tue, Dec 01, 2015 at 05:22:38PM +0200, Avi Kivity wrote:
>> On 12/01/2015 04:56 PM, Brian Foster wrote:
>>> On Tue, Dec 01, 2015 at 03:58:28PM +0200, Avi Kivity wrote:
>>>>>   io_submit() can probably block in a variety of
>>>>> places afaict... it might have to read in the inode extent map, allocate
>>>>> blocks, take inode/ag locks, reserve log space for transactions, etc.
>>>> Any chance of changing all that to be asynchronous?  Doesn't sound too hard,
>>>> if somebody else has to do it.
>>>>
>>> I'm not following... if the fs needs to read in the inode extent map to
>>> prepare for an allocation, what else can the thread do but wait? Are you
>>> suggesting the request kick off whatever the blocking action happens to
>>> be asynchronously and return with an error such that the request can be
>>> retried later?
>> Not quite, it should be invisible to the caller.
> I have a pony I can sell you.

You already sold me a pony.

>> That is, the code called by io_submit()
>> (file_operations::write_iter, it seems to be called today) can kick
>> off this operation and have it continue from where it left off.
> This is a problem that people have tried to solve in the past (e.g.
> syslets, etc) where the thread executes until it has to block, and
> then it's handled off to a worker thread/syslet to block and the
> main process returns with EIOCBQUEUED.

Yes, I remember that.

> Basically, you're asking for a real AIO infrastructure to
> beintroduced into the kernel, and I think that's beyond what us XFS
> guys can do...

Sure you can, Dave.  In fact you feel an irresistible urge to do it.

But I don't think the EIOCBQUEUED thing need be repeated.  We can have a 
simpler implementation:

  - Add a task flag TIF_AIO, which causes any new I/O to fail with 
EAIOWOULDBLOCK.

  - have __blockdev_direct_IO() do its block-mapping operations with 
TIF_AIO set (but remove it just before issuing the bio).

  - sys_aio_submit() catches EAIOWOULDBLOCK and resubmits the aio in a 
work item, this time without TIF_AIO games.

The effect would be similar to EIOCBQUEUED, but simpler, as instead of 
issuing any metadata I/O you abort the operation and restart it from 
scratch.

>
>>>>>   Reducing the frequency of block allocation/frees might also be
>>>>> another help (e.g., preallocate and reuse files,
>>>> Isn't that discouraged for SSDs?
>>>>
>>> Perhaps, if you're referring to the fact that the blocks are never freed
>>> and thus never discarded..? Are you running fstrim?
>> mount -o discard.  And yes, overwrites are supposedly more expensive
>> than trim old data + allocate new data, but maybe if you compare it
>> with the work XFS has to do, perhaps the tradeoff is bad.
> Oh, you do realise that using "-o discard" causes significant delays
> in journal commit processing? i.e. the journal commit completion
> blocks until all the discards have been submitted and waited on
> *synchronously*. This is a problem with the linux block layer in
> that blkdev_issue_discard() is a synchronous operation.....

I do now. What's the unicode for a crying face?

> Hence if you are seeing delays in transactions (e.g. timestamp updates)
> it's entirely possible that things will get much better if you
> remove the discard mount option. It's much better from a performance
> perspective to use the fstrim command every so often - fstrim issues
> discard operations in the context of the fstrim process - it does
> not interact with the transaction subsystem at all.
>
>

All right.  On the other hand we have to know when to issue it. That 
would be when nn% of the disk area have been rewritten.  Is there some 
counter I can poll every minute or so for this?  Not doing the fstrim in 
time would cause the disk performance to tank.

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