turn off lazyIO on disk files

Frank Heckenbach ih8mj at fjf.gnu.de
Thu Jul 3 18:22:33 CEST 2008


Wolfgang Helbig wrote:

> the program, Don Knuth's TeX adapted to GPC version 2.1, does heavy
> I/O on text files. I noticed a very high percentage of system time.
> I believe this is due to lazy I/O, which seems to be unbuffered.
> 
> GPC should apply lazy/IO  when the text file comes from the keyboard,
> but to use buffered I/O when it comes from the disk.
> 
> Is there a way to turn off lazy/IO on nonterminal text files?

GPC uses buffers on reading, but only reads as much data as are
available at any time. E.g., with a default buffer size of $4000, it
tries to read that many bytes when it needs more input, but if the
underlying read() system call returns less (e.g., because input from
a terminal, pipe, socket or other device has less bytes available),
it doesn't call read() again (to fill the buffer completely) until
more input is actually required. So there shouldn't be a problem
here (i.e., disk input should be fast because the buffer is usually
filled completely, while terminal input doesn't block
unwarrantedly).

On writing, GPC doesn't buffer yet at all; it's not implemented yet.
It's not that it couldn't be done, but it's more tricky, because GPC
supports various ways of seeking, pre-reading, getting the file
position etc., which all would have to take account of the buffers.

If it's a serious problem you could kludge it by installing a
user-defined file write routine which can do the buffering. If you
know that your application only does sequential writes, this would
work. I've done this once (see RewriteBuffer in cgi.pas in
http://fjf.gnu.de/misc/cgiprogs.tar.bz2). Let me know if you need
more details.

Frank

-- 
Frank Heckenbach, f.heckenbach at fh-soft.de, http://fjf.gnu.de/, 7977168E
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