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Date:      Fri, 31 Jan 97 15:27:13 +0000
From:      Andrew.Gordon@net-tel.co.uk
To:        "Ron G. Minnich" <rminnich@Sarnoff.COM>
Cc:        freebsd-hackers@freebsd.org
Subject:   Re(2): Re(2): Using rfork() / threads
Message-ID:  <"5fb2-970131152702-FFD1*/G=Andrew/S=Gordon/O=NET-TEL Computer Systems Ltd/PRMD=NET-TEL/ADMD=Gold 400/C=GB/"@MHS>
In-Reply-To: <Pine.SUN.3.91.970131093856.16303C-100000@terra>

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> well, i have another thing that was (ah well) offered to freebsd called 
> fastlock(). Fastlock was two things: 
> 1) test and set, which ran at memory bandwidth
> 2) a system call to be called only if the lock failed, 
>    which ran at system call speeds. 

Sounds just the ticket for what I need.

I am quite often writing servers for one network protocol or another,
where the the server holds per-user state for a large number of users,
but the users typically spend much of their time asleep at the keyboard
and hence a process-per-user design is wasteful, but in the absence
of async I/O, a one-process design has insufficient parallelism.
The ideal compromise is a small number of processes picking work off a
queue - but this needs the per-user state to be shared between
the processes and a means of sharing out the requests.

You can do some of this with SysV SHM and semaphores, but it gets
inefficient when one of the pieces of per-user state you would
like to share is an open fd.  rfork() and fastlock() seems a much
more efficient combination, though there is a potential problem with
running out of fds in a large system.



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