explore fuse optimizations

[stapelberg]

jacobsa/fuse implements fuse 7.12. Current Linux kernels are at 7.31. Here are a few features which we should look into to possibly improve performance:

• enable parallel dirops (FUSE_PARALLEL_DIROPS)
• permanent caching with invalidation: torvalds/linux@3b463ae (WIP in the notify branch)
• increase max_pages: torvalds/linux@5da784c
• symlink caching: torvalds/linux@5571f1e
• readdir caching: torvalds/linux@6433b89 (see next item)
• no opendir: torvalds/linux@d9a9ea9 (implicitly turns on readdir caching)
• no open: torvalds/linux@7678ac5 (implicitly turns on page cache)
• return nil with child inode == 0 instead of ENOENT to cache ENOENT: https://github.com/torvalds/linux/blob/2019fc96af228b412bdb2e8e0ad4b1fc12046a51/fs/fuse/dir.c#L353
• read from multiple file descriptors? https://john-millikin.com/the-fuse-protocol#multi-threading

We should also figure out a representative benchmark for root file system performance.

We can use bpftrace to measure the number of FUSE calls during builds:

sudo bpftrace -e 'kprobe:fuse_simple_request { @[kstack(perf)] = count(); }'

[stapelberg]

AFAICT, building pkgs/linux looks bottlenecked on the distri fuse process. Maybe improvements can be measured by compiling pkgs/linux and measuring the time difference.

edit: gtk+-2 might be a better candidate, as it completes in ≈2 minutes instead of ≈10 minutes