[dm-devel] [PATCH 1/9] QUEUE_FLAG_NOWAIT to indicate device supports nowait

[Jens Axboe]

On 08/10/2017 11:15 AM, Goldwyn Rodrigues wrote:
> 
> 
> On 08/10/2017 09:28 AM, Jens Axboe wrote:
>> On 08/10/2017 08:25 AM, Jan Kara wrote:
>>> On Thu 10-08-17 06:49:53, Goldwyn Rodrigues wrote:
>>>> On 08/09/2017 09:17 PM, Jens Axboe wrote:
>>>>> On 08/09/2017 08:07 PM, Goldwyn Rodrigues wrote:
>>>>>>>>>>>> No, from a multi-device point of view, this is inconsistent. I
>>>>>>>>>>>> have tried the request bio returns -EAGAIN before the split, but
>>>>>>>>>>>> I shall check again. Where do you see this happening?
>>>>>>>>>>>
>>>>>>>>>>> No, this isn't multi-device specific, any driver can do it.
>>>>>>>>>>> Please see blk_queue_split.
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> In that case, the bio end_io function is chained and the bio of
>>>>>>>>>> the split will replicate the error to the parent (if not already
>>>>>>>>>> set).
>>>>>>>>>
>>>>>>>>> this doesn't answer my question. So if a bio returns -EAGAIN, part
>>>>>>>>> of the bio probably already dispatched to disk (if the bio is
>>>>>>>>> splitted to 2 bios, one returns -EAGAIN, the other one doesn't
>>>>>>>>> block and dispatch to disk), what will application be going to do?
>>>>>>>>> I think this is different to other IO errors. FOr other IO errors,
>>>>>>>>> application will handle the error, while we ask app to retry the
>>>>>>>>> whole bio here and app doesn't know part of bio is already written
>>>>>>>>> to disk.
>>>>>>>>
>>>>>>>> It is the same as for other I/O errors as well, such as EIO. You do
>>>>>>>> not know which bio of all submitted bio's returned the error EIO.
>>>>>>>> The application would and should consider the whole I/O as failed.
>>>>>>>>
>>>>>>>> The user application does not know of bios, or how it is going to be
>>>>>>>> split in the underlying layers. It knows at the system call level.
>>>>>>>> In this case, the EAGAIN will be returned to the user for the whole
>>>>>>>> I/O not as a part of the I/O. It is up to application to try the I/O
>>>>>>>> again with or without RWF_NOWAIT set. In direct I/O, it is bubbled
>>>>>>>> out using dio->io_error. You can read about it at the patch header
>>>>>>>> for the initial patchset at [1].
>>>>>>>>
>>>>>>>> Use case: It is for applications having two threads, a compute
>>>>>>>> thread and an I/O thread. It would try to push AIO as much as
>>>>>>>> possible in the compute thread using RWF_NOWAIT, and if it fails,
>>>>>>>> would pass it on to I/O thread which would perform without
>>>>>>>> RWF_NOWAIT. End result if done right is you save on context switches
>>>>>>>> and all the synchronization/messaging machinery to perform I/O.
>>>>>>>>
>>>>>>>> [1] http://marc.info/?l=linux-block&m=149789003305876&w=2
>>>>>>>
>>>>>>> Yes, I knew the concept, but I didn't see previous patches mentioned
>>>>>>> the -EAGAIN actually should be taken as a real IO error. This means a
>>>>>>> lot to applications and make the API hard to use. I'm wondering if we
>>>>>>> should disable bio split for NOWAIT bio, which will make the -EAGAIN
>>>>>>> only mean 'try again'.
>>>>>>
>>>>>> Don't take it as EAGAIN, but read it as EWOULDBLOCK. Why do you say
>>>>>> the API is hard to use? Do you have a case to back it up?
>>>>>
>>>>> Because it is hard to use, and potentially suboptimal. Let's say you're
>>>>> doing a 1MB write, we hit EWOULDBLOCK for the last split. Do we return a
>>>>> short write, or do we return EWOULDBLOCK? If the latter, then that
>>>>> really sucks from an API point of view.
>>>>>
>>>>>> No, not splitting the bio does not make sense here. I do not see any
>>>>>> advantage in it, unless you can present a case otherwise.
>>>>>
>>>>> It ties back into the "hard to use" that I do agree with IFF we don't
>>>>> return the short write. It's hard for an application to use that
>>>>> efficiently, if we write 1MB-128K but get EWOULDBLOCK, the re-write the
>>>>> full 1MB from a different context.
>>>>>
>>>>
>>>> It returns the error code only and not short reads/writes. But isn't
>>>> that true for all system calls in case of error?
>>>>
>>>> For aio, there are two result fields in io_event out of which one could
>>>> be used for error while the other be used for amount of writes/reads
>>>> performed. However, only one is used. This will not work with
>>>> pread()/pwrite() calls though because of the limitation of return values.
>>>>
>>>> Finally, what if the EWOULDBLOCK is returned for an earlier bio (say
>>>> offset 128k) for a 1MB pwrite(), while the rest of the 7 128K are
>>>> successful. What short return value should the system call return?
>>>
>>> This is indeed tricky. If an application submits 1MB write, I don't think
>>> we can afford to just write arbitrary subset of it. That just IMHO too much
>>> violates how writes traditionally behaved. Even short writes trigger bugs
>>> in various applications but I'm willing to require that applications using
>>> NOWAIT IO can handle these. However writing arbitrary subset looks like a
>>> nasty catch. IMHO we should not submit further bios until we are sure
>>> current one does not return EWOULDBLOCK when splitting a larger one...
>>
>> Exactly, that's the point that both Shaohua and I was getting at. Short
>> writes should be fine, especially if NOWAIT is set. Discontig writes
>> should also be OK, but it's horrible and inefficient. If we do that,
>> then using this feature is a net-loss, not a win by any stretch.
>>
> 
> To make sure I understand this, we disable bio splits for NOWAIT bio so
> we return EWOULDBLOCK for the entire I/O.

That's also not great, since splits is a common operation, and the majority
of splits can proceed without hitting out-of-resources. So ideally we'd
handle that case, but in a saner fashion than the laissez faire approach
that the current patchset takes.

-- 
Jens Axboe