Lasse Collin wrote:
Phillip Lougher wrote:
Currently, as mentioned above, Squashfs decompresses into a single
contiguous output buffer. But, due to the linux kernel mailing
list's dislike of vmalloc, this is being changed. In future Squashfs
will decompress into a sequence of 4 KiB output buffers (possibly in
the page cache).
To my understanding, using 4 KiB output buffers can make sense only if
the dictionary size is significantly smaller than the Squashfs block
size. This is because an output buffer scattered to 4 KiB pieces means
that the the dictionary has to be vmalloced as part of the LZMA decoder
state.
For example, if the dictionary size is equal to the Squashfs block size,
the same amount of memory that earlier Squashfs versions vmalloced for
the output buffer is now vmalloced by the uncompression code for the
dictionary. Plus, memcpy is needed to get the data from the dictionary
to the 4 KiB output buffers.
The issue that moving to 4 KiB output buffers solves is it reduces
significantly the number of 1 MiB (or 128 KiB for the default block
size) buffers that need to be vmalloced. Squashfs caches the last 3
fragment buffers read, and moving to 4 KiB buffers eliminates these
vmallocs.
Obviously moving to 4 KiB output buffers will require a 1 MiB dictionary
workspace to be vmalloced, but this is still much less than the 3
buffers that currently need to be vmalloced.
Phillip
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