Re: [PATCH 00/23] Adaptive read-ahead V11

[Wu Fengguang <wfg@mail.ustc.edu.cn>]

On Sun, Mar 19, 2006 at 10:53:46AM -0500, Jon Smirl wrote:
> In another thread someone made a mention that this problem may have
> something to do with the pools of memory being used for sendfile. The
> readahead from sendfile is going into a moderately sized pool. When
> you get 100 of them going at once the other threads flush the
> readahead data out of the pool before it can be used and thus trigger
> the thrashing seek storm. Is this true, that sendfile data is read
> ahead into a fixed sized pool? If so, the readahead algorithms would

The pages are kept in a cache pool which is made of all the free memory.
E.g. the following command shows a system with 331M cache pool:

% free -m
             total       used       free     shared    buffers     cached
Mem:           488        482          5          0          7        331
-/+ buffers/cache:        142        345
Swap:          127          0        127

That would be more than enough for the stock read-ahead to handle 100
concurrent readers.

> need to reduce the sendfile window sizes to stop the pool from
> thrashing.

Sure, it is the desired behavior. This patch provides exactly this feature :)

Cheers,
Wu