Re: open a large file in win64

From:

"Tom Widmer [VC++ MVP]" <tom_usenet@hotmail.com>

Newsgroups:

microsoft.public.vc.language

Date:

Tue, 06 Mar 2007 12:37:36 +0000

Message-ID:

<uzbM6w#XHHA.4868@TK2MSFTNGP03.phx.gbl>

Mycroft Holmes wrote:

I think the extra time might be due to Map/UnmapViewOfFile... is that
possible?

In principle we could use, say, fread and copy a fixed amount of bytes to
some buffer. Is that the preferred way?

For an application that does one pass through a file in purely sequential
order, memory mapping is not a win. You would be much better off to use
ReadFile or fread and get some buffering.

Consider what you're asking. You map in an entire gigabyte. Then, when
you're done with that one and move to the next one, that entire gigabyte
becomes free memory, and it has to map in the SECOND entire gigabyte
before
it can continue. With the normal ReadFile routines, the operating system
will be reading ahead so that you overlap your activity with the disk to
the maximum extent possible.
--

My experiments seem to leadto the opposite conclusion:
I tried fread'ing chunks of 64K and chunks of 16MB, it takes more than 400
seconds anyway... so I'm still confused.

Mapviewoffile seems the fastest way to read, but the utility I downloaded is
much faster...

As far as I know, ReadFile on a file opened with FILE_FLAG_NO_BUFFERING
| FILE_FLAG_SEQUENTIAL_SCAN is the fastest technique for sequential
reading of a large file. Make sure you read in chunks of at least 64KB
(and a multiple of the sector size of the disk), and meet the other
requirements of FILE_FLAG_NO_BUFFERING (e.g. alignment).

Tom

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