Re: [PATCH 04/10] AXFS: axfs_inode.c

Jared Hulbert
2008-08-22T03:27:52Z

> I assume compressed blocks can be larger than PAGE_CACHE_SIZE? This suffers
> from the rather obvious inefficiency that you decompress a big block >
> PAGE_CACHE_SIZE, but only copy one PAGE_CACHE_SIZE page out of it. If
> multiple files are being read simultaneously (a common occurrence), then
> each is going to replace your one cached uncompressed block
> (sbi->current_cnode_index), leading to decompressing the same blocks over
> and over again on sequential file access.
>
> readpage file A, index 1 -> decompress block X
> readpage file B, index 1 -> decompress block Y (replaces X)
> readpage file A, index 2 -> repeated decompress of block X (replaces Y)
> readpage file B, index 2 -> repeated decompress of block Y (replaces X)
>
> and so on.

Yep. Been thinking about optimizing it. So far it hasn't been an
issue for my customers. Most fs traffic being on the XIP pages. Once
I get a good automated performance test up we'll probably look into
something to improve this.