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From Bill Stoddard <b...@wstoddard.com>
Subject Re: [WIN32] alternative apr_pollset implementation proposal
Date Tue, 19 Apr 2005 14:04:55 GMT
Bill Stoddard wrote:
> Mladen Turk wrote:
> 
>> Hi,
>>
>> Since the WIN32 imposes pretty nasty limit on FD_SETSIZE to 64, that
>> is way too low for any serious usage, I developed an alternative
>> implementation.
>> Also the code support the APR_POLLSET_THREADSAFE flag.
>>
>> A simple patch to apr_arch_poll_private.h allows to have
>> multiple implementations compilable.
>>
>> Any comments?
>>
>> Regards,
>> Mladen.
> 
> 
> Brain dump...
> 
> It may be possible to use IOCompletionPorts on Windows to implement 
> apr_pollset_*.  IOCPs aare very scalable but moving to IOCPs will 
> require a complete rewrite of the apr_socket implementation on Windows. 
> And there is the small matter of a simple technical issue that needs to 
> be investigated...
> 
> IOCPs support true async network i/o. BSD KQ, Solaris /dev/poll, epoll 
> et. al. are not async, they are event driven i/o models. When you issue 
> an async read on Windows, the kernel will start filling your i/o buffer 
> as soon as data is available. With event driven i/o, the kernel tells 
> you when you can do a read() and expect to receive data. See the 
> difference? Your buffer management strategy will be completely different 
> between async i/o and event driven i/o and I am not sure how APR (or 
> applications that use APR) can be made to support both cleanly. 
One thought on the buffer management issue... rather than managing buffers, we manage 'i/o
objects' that 
contain references to network i/o buffers (among other things). These i/o objects have their
own scope and 
lifetime. When an i/o is issued and it does not complete immediately, we place the i/o object
into a 
container, searchable by a key. The application should not reference the i/o object further
once it is in the 
container. I know IOCPs enable passing a key on the i/o call that the kernel will return on
the IOCP 
notification. I am sure something similar is available on Unix.

When an app receives notification that io is complete (or can be done with the expectation
that it will 
complete), we find the i/o object in the container, and issue read passing the i/o object
on the call. Under 
the covers, the read on windows just returns the buffer in the i/o object. On unix, the read
is issued to 
fetch the bytes from the kernel. The buffer management is hidden in the i/o object.

Bill


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