read vs. mmap (or io vs. page faults)

[Julian Elischer]

On Sun, 20 Jun 2004, Matthew Dillon wrote:

>     Hmm.  Well, you can try calling madvise(... MADV_WILLNEED), that's what
>     it is for.  
> 
>     It is usually a bad idea to try to populate the page table with all
>     resident pages associated with the a memory mapping, because mmap()
>     is often used to map huge files... hundreds of megabytes or even 
>     dozens of gigabytes (on 64 bit architectures).  The last thing you want
>     to do is to populate the page table for the entire file.  It might
>     work for your particular program, but it is a bad idea for the OS to
>     assume that for every mmap().
> 
>     What it comes down to, really, is whether you feel you actually need the
>     additional performance, because it kinda sounds to me that whatever 
>     processing you are doing to the data is either going to be I/O bound,
>     or it isn't going to run long enough for the additional overhead to matter
>     verses the processing overhead of the program itself.
> 
>     If you are really worried you could pre-fault the mmap before you do
>     any processing at all and measure the time it takes to pre-fault the
>     pages vs the time it takes to process the memory image.  (You pre-fault
>     simply by accessing one byte of data in each page across the mmap(),
>     before you begin any processing).

pre-faulting is best done by a worker thread or child process, or it
will just slow you down..


> 
> 					-Matt
> 					Matthew Dillon 
> 					<dillon at backplane.com>
> 
> :=     It's hard to say.  mmap() could certainly be made more efficient, e.g.
> :=     by faulting in more pages at a time to reduce the actual fault rate.
> :=     But it's fairly difficult to beat a read copy into a small buffer.
> :
> :Well, that's the thing -- by mmap-ing the whole file at once (and by
> :madvise-ing with MADV_SEQUENTIONAL), I thought, I told, the kernel
> :everything it needed to know to make the best decision. Why can't
> :page-faulting code do a better job using all this knowledge, than the
> :poor read, which only knows about the partial read in question?
> :
> :I find it so disappointing, that it can, probably, be considered a bug.
> :I'll try this code on Linux and Solaris. If mmap is better there (as it
> :really ought to be), we have a problem, IMHO. Thanks!
> :
> :	-mi
> :
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