Re: Beagle CPU usage (was Proposed module: tracker)

On Wed, 2007-03-28 at 00:28 +0200, Xavier Bestel wrote:
> On mar, 2007-03-27 at 14:52 -0600, Federico Mena Quintero wrote:

> > [/me wonders if thumbnailers are measurably faster by using mmap()
> > instead of read()...]

> It depends on their access pattern. If they need sequential access,
> read() is better because it can do prefetching (readahead).

I think what Federico was getting at is that for most thumbnailers,
rendering is likely to be a bigger bottleneck than I/O on a local file
system. But we don't know this for sure until we've measured it.

mmap() can do prefetching too - and it will likely load a 4k page at a
time anyway. So I doubt prefetch plays a role in a comparison of read()
and mmap().

I tend to favor read() on user data:

- For robustness: An mmap() map can become invalid if the file is
truncated, crashing the process.
- Because mmap() makes your VSize - and with accesses, your RSS - grow
correspondingly, with subsequent bug reports from irate users.

Hans Petter

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