Re: [RFC] adding aio_readv/writev

[Shailabh Nagar <nagar@watson.ibm.com>]

Clement T. Cole wrote:

>>>Comments, reasons for not doing async readv/writev directly welcome.
>>>
>
>How about the case for it...  See Pages 404-406 [Section 12.7] of
>Richard Steven's ``Advanced Programming in the Unix Environment''
>[aka APUE].  Richard measures almost a factor of 2 difference
>in system time between using vectored I/O and not using it on
>a Sun and on a x86.
>
It would have been nice to have corresponding data for the async path.

><snip>
>
>So... let's get back to the basic issue....
>
>We know that vectored/scatter gather I/O can help a number of real
>applications ... Richard demonstrated that.  We have some examples
>[like DB2] that have use vectored I/O successfully.  We also
>know asynchronous I/O has been demonstrated to be useful and
>know that some commerical folks have used that.  
>
>I'm gather from some of the comments, adding async/vectored
>will make an already complex subsystem, even more so [i.e. not
>a resounding endorsement for sure this is easy].
>

I wouldn't say so. Adding async vectored I/O to the 2.5 code won't make 
it more complex since the underlying functions
do handle iovec's anyway.

>
>
>So the question is can async vectored I/O be implemented 
>to have a positive gain, such as it did within the traditonal one.
>If the complexity is too high and it does not help much...then
>maybe this is a Chimera to leave alone.   But.... if it can be
>done with some level of elegance... well.... the past history is
>that the commerical folks have used those features.
>

It seems to be a case of "complexity is low, benefits are unknown". I 
guess the best thing is to develop a patch and see what people think 
about the complexity part. The benefits part will become clear only when 
the async interfaces are reasonable functional and we can compare the 
following

- call async readv directly
vs
- multiple calls to io_submit using one iocb (each call corresponds to 
one element of user's vector)
vs
- single call to io_submit using multiple iocb's (each iocb corresponds 
to one element of user's vector)

Since the raw/O_DIRECT interfaces offer asynchrony (through Badari 
Pulavarty & Mingming Cao's patches), it should be possible to test this 
out.

More on this shortly,
- Shailabh