Date: Sat, 17 Apr 1999 02:52:56 +0200 (MET DST) From: Olaf Wagner <wagner@luthien.in-berlin.de> To: freebsd-current@freebsd.org Subject: Re: cvsup Message-ID: <199904170052.CAA31420@luthien.iceflower.in-berlin.de> In-Reply-To: <XFMail.990413162255.jdp@polstra.com>
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In article <XFMail.990413162255.jdp@polstra.com> you wrote:
> Daniel Eischen wrote:
>> John Polstra wrote:
>>
>>> My hunch is that it's not a fairness issue. It's just the fact that
>>> when you block in disk I/O, the whole process (all threads) blocks.
>>
>> That statement made me think that Modula-3 had it's own threading
>> support because our native threads using non-blocking file I/O.
> For disk I/O? Are you sure? If so then it must use the aio/lio calls
> or something similar. Disk I/O calls _always_ block, even if you've
> set O_NONBLOCK on the file descriptor.
The SchedulerPosix interface in m3core exports the procedures
IOWait() and IOAlertWait(), which can be used to wait for
a single file descriptor without stopping other threads.
These procedures are used in FilePosix.IntermittentRead to
implement non-blocking reading. The default implementation
does not count RegularFiles as intermittent readers and
writers though:
REVEAL
Pipe.T = IntermittentFile BRANDED OBJECT END;
Terminal.T = IntermittentFile BRANDED OBJECT END;
RegularFile.T = RegularFile.Public BRANDED OBJECT OVERRIDES
read := RegularFileRead;
write := RegularFileWrite;
seek := RegularFileSeek;
flush := RegularFileFlush;
lock := RegularFileLock;
unlock := RegularFileUnlock
END;
So read calls on files always block until the Unix read call
returns; no other threads can run.
If the O_NONBLOCK flag on fds would work for disk I/O in FreeBSD,
then the code from the intermittent read procedures in FilePosix
could easily be used to let the scheduler run other threads
while one thread is waiting for data from a file. As John Polstra
pointed out, this seems not to be possible...
Aio could probably be used, but there seems to be a shortage of
man pages about aio calls in FreeBSD and I haven't got my Posix
book at hand, so I'm not sure how difficult this would be.
--
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