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Date: Wed, 5 Nov 2003 20:01:11 +0300 (MSK)
From: Igor Sysoev <is@rambler-co.ru>
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To: Robert Watson <rwatson@freebsd.org>
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On Wed, 5 Nov 2003, Robert Watson wrote:

> On Wed, 5 Nov 2003, Igor Sysoev wrote:
> 
> > On Wed, 5 Nov 2003, Robert Watson wrote:
> > 
> > > On Wed, 5 Nov 2003, Igor Sysoev wrote:
> > > 
> > > > As to worker kthreads I think it's better to queue aio operation as it
> > > > was made in src/sys/kern/vfs_aio.c:aio_qphysio(). 
> > > 
> > > One of the things that worries me about the proposal to use kernel worker
> > > threads to perform the I/O is that this can place a fairly low upper bound
> > > on effective parallelism, unless the kernel threads themselves can issue
> > > the I/O's asynchronously.  In the network stack itself, we are event and
> > > queue driven without blocking--if we can maintain the apparent semantics
> > > to the application, it would be very nice to be able to handle that at the
> > > socket layer itself.  I.e., not waste a thread + stack per "in-progress" 
> > > operation, and instead have a worker or two that simply propel operations
> > > up and down the stack (similar to geom_up and geom_down). 
> > 
> > As far as I understand src/sys/kern/vfs_aio.c:aio_qphysio() (that
> > handles AIO on raw disks) does not use kthreads and simply queues
> > operations. 
> 
> I think it sounds like we're actually agreeing with each other. 

Yes.

> Currently, AIO does use threads for non-character devices, so in the
> socket case it will be using a worker thread. 

Yes, current AIO implementation uses worker threads for regular files.
But I suggest not to use AIO code for sendfile() to read a file page.
I suggest to use BUF_STRATEGY() like aio_qphysio() does.

Is it possible to read a regular file page using BUF_STRATEGY() ?


Igor Sysoev
http://sysoev.ru/en/