Comment 8 for bug 1636847

Adding some notes and ideas based on looking more at the squashfs implementation.

One idea I had was to reduce the data cache (or maybe even eliminate it) by making squashfs decompress directly into the page cache. It turns out that squashfs already does this if CONFIG_SQUASHFS_FILE_DIRECT=y, which is the case in our kernel. Rather it tries to read into the page cache directly first, and if that fails it falls back to using the cache, so we can't eliminate the cache entirely. But it does mean that squashfs can probably get by with much less data cache than it currently allocates for SQUASHFS_DECOMP_MULTI and SQUASHFS_DECOMP_MULTI_PERCPU when that option is set. We can't reduce the data cache for CONFIG_SQUASHFS_DECOMP_SINGLE since it always needs enough cache for at least one uncompressed data block.

Also important to note is that although the squashfs data cache size is determined by which decompressor implementation is selected, the cache and decompressor states are managed independently, therefore there's no requirement that they are so tightly coupled. The coupling makes sense if SQUASHFS_FILE_DIRECT is disabled but seems less sensible if it's enabled.

The decompressor implementations come with their own tradeoffs:

- CONFIG_SQUASHFS_DECOMP_SINGLE: Uses less memory. However decompression is serialized, i.e. for a given superblock only one block can be decompressed at a time.

- SQUASHFS_DECOMP_MULTI: Allows for (num_online_cpus() * 2) parallel decompressions, which means the number of parallel decompressions could vary depending on how many CPUs were online when the filesystem was mounted. Also allocates the same number of blocks for the data cache. This implementation has more overhead associated with managing decompressor state memory than the others.

- SQUASHFS_DECOMP_MULTI_PERCPU: This maintains per-cpu decompressor state, therefore it is lockless and there is no overhead for managing the decompressor state memory. That means it uses more RAM though, both for decompressor state memory and for data cache. You also can have as many parallel decompressions going on as there are CPU cores.

Based on this, here are a couple of ideas we could try to strike a good balance between performance and RAM usage:

1. Add a config option for the maximum number of data cache blocks. This would allow us to use one of the implementations which allows for parallel decompression without the data cache size exploding. As long as most blocks get decompressed directly into the page cache (which we'll need to verify) having only one or two blocks in the data cache should not be detrimental to performance.

2. Make a new decompressor implementation which allows for full customization of the number of parallel decompressions and number of blocks in the data cache. In my opinion SQUASHFS_DECOMP_SINGLE is too little parallelization but the others are too much on systems with large numbers of CPUs. Instead we could specify both the maximum number of parallel decompressions in the kernel config (possibly capped at the number of possible CPU cores) and the number of data blocks to cache to suit our needs.