Q46741: Possible Cause for Slow Stream I/O

Article: Q46741
Product(s): See article
Version(s): 5.10
Operating System(s): MS-DOS
Keyword(s): ENDUSER | S_QuickC | mspl13_c
Last Modified: 25-JUL-1989

Question:

I have an application program that does intensive file manipulation
with the stream level I/O routines. Normally it works very fast, but
lately the file operations have become increasingly slower and now it
can take several minutes to process a single file. What causes this
time delay in the stream I/O routines and how can I prevent it?

Response:

The stream level I/O routines inherit their speed and flexibility from
the buffering system they use. Upon opening a file with the fopen
function, a file record is created that contains pointers into a
stream buffer. This buffer is then allocated (malloc-ed) when the
first I/O operation is performed. If there is not enough room in the
heap for this buffer allocation, the file operation continues with a
buffer size of one character.

A stream I/O routine that takes an excessive amount of time is most
likely the result of a failure to allocate the 512-byte buffer on the
heap (near or far, depending on the memory model). Without this
buffer, the I/O routines are extremely slow, requiring disk access for
all I/O operations.

To get around this problem, you can do one of two things. Reduce the
amount of allocation on the heap. Or, change memory models to compact
or large (if you are in small or medium). If you change memory models,
you may need to use the /Gt switch to push static data out of DGROUP.

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