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From: John Baldwin <jhb@freebsd.org>
To: freebsd-current@freebsd.org
Subject: Re: [CFT] new sendfile(2)
Date: Tue, 18 Feb 2014 13:28:26 -0500
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Cc: Gleb Smirnoff <glebius@freebsd.org>, David Chisnall <theraven@freebsd.org>,
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On Monday, February 17, 2014 6:24:21 am David Chisnall wrote:
> P.S. If aio() is creating a new thread per request, rather than scheduling 
them from a pool, then that is also likely a bug.  The aio APIs were designed 
so that systems with DMA controllers could issue DMA requests in the syscall 
and return immediately, then trigger the notification in response to the DMA-
finished interrupt.  There shouldn't need to be any kernel threads created to 
do this...

AIO uses a pool, but the requests are all done synchronously from that
pool.  While our low-level disk routines are async (e.g. GEOM etc.),
the filesystem code above that generally is not.  The aio code does have
some special gunk in place for sockets (and I believe raw disk I/O) to
make it truly async, but aio for files uses sychronous I/O from a pool
of worker threads.

-- 
John Baldwin