Re: [PATCH] tcp: do not promote SPLICE_F_NONBLOCK to socket O_NONBLOCK

[Evgeniy Polyakov]

On Fri, Jul 18, 2008 at 05:04:17PM +0300, Octavian Purdila (opurdila@xxxxxxxxxxx) wrote:
> > It will block in sending and/or other than network reading. With your
> > patch if receiving socket was opened in blocking mode, than there is no
> > way to finish splice-in until whole requested number of bytes are read.
> > SPLICE_F_NONBLOCK is an extension, consider it like recv() with temporal
> > non-blocking flag.
> 
> The way is see it, and API and documentation is written, SPLICE_F_NONBLOCK was 
> added only for choosing to block or not block on the _pipe_, not on the other 
> fd. 
> 
> And, IMHO, it should be kept that way. If we need to make certain splice 
> operations non-blocking for the other file descriptor, then maybe we should 
> add a separate flag for that. But, again IMHO, overloading SPLICE_F_NONBLOCK 
> with responsabilities for both the pipe and the other file descriptor is 
> wrong is at is taking the freedom from the application of controlling things. 
> 
> And when you do that, sooner or later you will run in a scenario which 
> requires workarounds in the applications to bypass the API assumptions. 

Why? There is clearly documented behaviour of the call, it works exactly
like it is supposed to work - it tries to be non-blocking everywhere
where it can, but not always, that's why there is a sentence which
states that even with given flag call may block.

> > > Sorry, it was an unfortunate example :) This is not about not enough data
> > > being available. Lets change the number of packets in the example with 20
> > > instead of 16 (and keep the size to 17) - the splice call will still
> > > block because of the pipe being full. The pipe can only hold PIPE_BUFFERS
> > > packets (which is 16 currently).
> >
> > Why? It will read its data from 16 packets, then send them into another end
> > of the pipe :)
> >
> 
> splice will consume one packet at a time and will try to feed them in the 
> pipe. Since the pipe can only hold 16 descriptors, on the 17th it will block. 

If there are 20 packets in the queue it will get 16 and put them into
another end (in the next call in your example). Where will it block?

> > You propose to change a very useful splice feature (actually you would just
> > remove it at all with the same results for reading network path, since
> > it is essentially what you did :) - not to block when it is possible.
> >
> > This kind of non-blocking mode was added for performance issues too:
> > consider application which reads from the network and writes into the
> > file, while there is no data in the socket it can write what was already
> > read into any object attached to the given end of the pipe.
> 
> I have my doubts about the benefit of using the non-blocking operations only 
> for some splice calls :) But to solve both issues the solution would be to 
> add a separate non-blocking flag for the other file descriptor. Is that ok?

I really do not think that there is any kind of problem with current
behaviour, and thus there is no need to introduce additional flags
and/or change existing behaviour, but I can understand you that existing
approach does not met your expectation, so you are trying to change it.
I've added Jens Axboe to copy list, who is responsible for splice
design.

Btw, you are also trying to change existing userspace API, which may be
very much forbidden at this stage.

-- 
	Evgeniy Polyakov
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