On 2025/7/17 10:49, Eric Biggers wrote:
On Wed, Jul 16, 2025 at 11:37:28PM +0100, Phillip Lougher wrote:
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buffer. I suspect that vmap() (or vm_map_ram() which is what f2fs uses) is actually more efficient than these streaming APIs, since it avoids the internal copy. But it would need to be measured.
Of course vm_map_ram() (that is what erofs relies on first for decompression in tree since 2018, then the f2fs one) will be efficient for decompression and avoid polluting unnecessary caching (considering typical PIPT or VIPT.) Especially for large compressed extents such as 1MiB, another memcpy() will cause much extra overhead over lz4. But as for gzip, xz and zstd, they just implement internal lz77 dictionaries then memcpy for streaming APIs. Since those algorithms are relatively slow (for example Zstd still relies on Huffman and FSE), I don't think it causes much difference to avoid memcpy() in the whole I/O path (because Huffman tree and FSE table are already slow), but lz4 matters. Thanks, Gao Xiang
