>Alright, so I investigated this question a little bit more:
>Test: 102,400,000 Megabytes to IDE disk, with Pentium 166 or something like
>that.
>So the timings with mmap:
>Write: ~28 sec
>Reading: ~20 sec
>
>With read/write, the speed-up stronlgly depends on how many bytes you write to
>the disk at once:
>
>Write, 4 bytes (int): ~190 sec
>Read, 4 bytes : ~ 90 sec
>
>Write, 1024 bytes: ~20-22 sec (this tends to be faster than mmap)
>Read, 1024 bytes: ~20 sec
>I experimented with a few write-lengths here, but it varies a lot because this
>is a server, so there is not much use of giving lots of timings here. They
>were usually 20-30 secs. And it's also hard to make sure that the data from the
>prev read/write are not in the swap/cache. And i don't have more time to
>experiment with this. It was interesting, but now let's speed-up the X:-).
>It still might be a good idea to use mmap. You can't really guarantee in many
>cases that you can read long blocks at the same time, and with mmap, you just
>have to use it as a memory copy virtually.
>Ferenc
Make certain that you don't miss the point of mmap. It's strongest
feature is that if you DO NOT WRITE to the datafile it saves a lot. I
think the original question was asking about input files?
If you never have to write to the file mmap it as read only. By doing
this you allow the system to only worry about paging out the data. It
just deletes the page. If you make it read write then it needs to
swap check the page and save out any modified data.
The point that was raised about compatability is a very important
point that is sort of being overlooked as well. Your data input files
need to be in the correct format for the architecture you're running
on. That is all of the floats/int must by byte aligned propperly,
lots of RISC machines have funny requirements here. There's also the
problem that binary data may change between different processors, best
example is floating point.
--------------------------------------------------------------
Robert E. Anderson email: [EMAIL PROTECTED]
Systems Programmer phone: (603) 862-3489
UNH Research Computing Center fax: (603) 862-1915
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