Re: readahead(2) - Linux

On Mon, 3 Mar 2008, Julian Elischer wrote:

> Ivan Voras wrote:
>> Ed Schouten wrote:
>>> * carlos neira <[e-mail]> wrote:
>>>> is there an equivalent of readahead syscall in linux , for freebsd ?. i 
>>>> was looking at http://preload.sourceforge.net/ , and it needs this .
>>> Maybe a mmap(), followed by a madvise()?
>> 
>> Or an open() followed by a read() loop? :) If the goal is to preload the 
>> files, this one is certainly going to do it :)
>
> the aim is to load it into system memory but not copy anything into user 
> memory.

In an ideal world (tm), a prefetch system call doesn't actually force the I/O 
to happen, it just hints that if it did happen, life would then be better. 
Then, in said ideal world (tm), the VM system can juggle investing pages in 
memory and I/O capacity in heuristic read-ahead, prefetch hints from the 
application, anonymously process memory, and buffer cache, based on what is 
most effective for particular applications or workloads.  Last time I read up 
on I/O prefetching literature, it was considered quite difficult to place 
prefetch calls in applications in a useful way, and that normal heuristic 
read-ahead, which we already support, actually caught a high percentage of 
real application cases since applications do tend to order and store data 
usefully in files.  For certain applications, though, that doesn't help much, 
and there isn't a way to tune "up" read-ahead prefetching, so it could be 
we're no longer doing as good a job.

Robert N M Watson
Computer Laboratory
University of Cambridge
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