simpler I/O buffering [was: RE: An IO Question from a Newbie]
heringto at cs.unc.edu
Mon Sep 15 15:50:13 EDT 2003
I've long thought that I/O buffering behavior--not just in Haskell, but
in most places I've encountered it--was unnecessarily complicated.
Perhaps it could be simplified dramatically by treating it as strictly a
performance optimization. Here's a sketch of the approach.
Writing a sequence of characters across the interface I'm proposing is a
request by the writing program for those characters to appear at their
destination "soon". Ideally, "soon" would be "immediately"; however, the
characters' appearance may deliberately be delayed ("buffered"), for
efficiency, as long as such delay is "unobtrusive" to a human user of the
program. Buffering timeouts would depend on the device; for a terminal,
perhaps 50-100 ms would be appropriate. Such an interval would tend not
to be noticeable to a human user but would be long enough to effectively
collect, say, an entire line of output for output "in one piece". The use
of a reasonable timeout would avoid the confusing behavior where a
newline-less prompt doesn't appear until the prompted data is entered.
With this scheme, I/O buffering no longer has any real semantic content.
(In particular, the interface never guarantees indefinite delay in
outputting written characters. Line buffering, if semantically important,
needs to be done above the level of this interface.) Hence, buffering
control could be completely eliminated from the interface. However, I
would retain it to provide (non-semantic) control over buffering. The
optional buffer size currently has such an effect. A timeout value could
be added for fine tuning. (Note that a timeout of zero would have an
effect similar to Haskell's current NoBuffering.) Lastly, the "flush"
operation would remain, as a hint that it's not worth waiting even the
limited timeout period before endeavoring to make the characters appear.
Is such an approach feasible? Has it been implemented anywhere? Would
such behavior best be implemented by the operating system? Could it be
implemented by the runtime system?
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