Re: Heuristic readahead for filesystems

[Hans Reiser <reiser@namesys.com>]

Davide Libenzi wrote:

>On Wed, 11 Sep 2002, Xuan Baldauf wrote:
>
>  
>
>>Hello,
>>
>>I wonder wether Linux implements a kind of heuristic
>>readahead for filesystems:
>>
>>If an application reads a directory with getdents() and if
>>in the past, it stat()ed a significant part of the directory
>>entries, it is likely that it will stat() every entry of
>>every directory it reads with getdents() in the future. Thus
>>readahead for the stat data could improve the perfomance,
>>especially if the stat data is located closely to each other
>>on disk.
>>
Nikita should comment on this.

>>
>>If an application did a stat()..open()..read() sequence on a
>>file, it is likely that, after the next stat(), it will open
>>and read the mentioned file. Thus, one could readahead the
>>start of a file on stat() of that file.
>>
>>Combined: If an application walks a directory tree and
>>visits each file, it is likely that it will continue up to
>>the end of that tree.
>>    
>>
>
>M$ Win XP does exactly something like this and keep applications
>( windows\prefetch ) and boot profiles that it uses to prefetch disk data
>and avoid long page fault latencies. It does kind-of-work but care should
>be taken adopting a similar technique on Linux ( patents ).
>
>
>
>- Davide
>
>
>-
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>  
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There was a Usenix paper on profiling based prefetching, does it predate 
the MS patents?

Rik, the more reads you put into the elevator, the more efficiently it 
can sort the reads, so even if latency reduction is not important, 
prefetching can still help (in principle).

Hans