Re: ReadFile vs fread

"Scott McPhillips [MVP]" <org-dot-mvps-at-scottmcp>
Thu, 8 Jan 2009 22:55:32 -0500
"James Williams" <> wrote in message

What I do with the file is read in a minimum of 32KBytes. The read is
aligned to a disk sector size. I have X number of threads operating on
the same file with the same file handle. Each thread is given a section
of the file to work with. I.E. Thread 1 get the first 10Mbyte, thread 2
gets the next 10Mbyte plus any slop do to uneven division of file size to
number of buffer.

Then the file is processed in parallel. I have been using just fread and
using a critical section around the read and fsetpos for reposition for
the given thread. This seems to be working good. However, I would like
to get away from the CRT as much as possible.

Does this seem like a reasonable way of doing this.


It seems unlikely that the disk read would go faster by doing it from
multiple threads, since you're adding overhead with the seek, and of course
the disk can't read from multiple places concurrently. But testing is the
only way to be sure.

You can improve read performance by using the FILE_FLAG_NO_BUFFERING option
in CreateFile, although it imposes very specific requirements on how you can
allocate the buffers and read. And further performance increases can be had
with FILE_FLAG_OVERLAPPED, which will let your code start processing the
first buffer while the second buffer is being read.

Scott McPhillips [VC++ MVP]

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