Tune zio buffer caches and their alignments

[amotin]

We should not always use PAGESIZE alignment for caches bigger than it and SPA_MINBLOCKSIZE otherwise. Doing that caches for 5, 6, 7, 10 and 14KB rounded up to 8, 12 and 16KB respectively make no sense. Instead specify as alignment the biggest power-of-2 divisor. This way 2KB and 6KB caches are both aligned to 2KB, while 4KB and 8KB are aligned to 4KB.

Reduce number of caches to half-power of 2 instead of quarter-power of 2. This removes caches difficult for underlying allocators to fit into page-granular slabs, such as: 2.5, 3.5, 5, 7, 10KB, etc. Since these caches are mostly used for transient allocations like ZIOs and small DBUF cache it does not worth being too aggressive. ~7/8 of space efficiency on average should be sufficient. Due to the above alignment issue some of those caches were not working properly any way. 6KB cache now finally has a chance to work right, placing 2 buffers into 3 pages, that makes sense.

Remove explicit alignment in Linux user-space case. I don't think it should be needed any more with the above fixes.

As result on FreeBSD instead of such numbers of pages per slab:

vm.uma.zio_buf_comb_16384.keg.ppera: 4
vm.uma.zio_buf_comb_14336.keg.ppera: 4
vm.uma.zio_buf_comb_12288.keg.ppera: 3
vm.uma.zio_buf_comb_10240.keg.ppera: 3
vm.uma.zio_buf_comb_8192.keg.ppera: 2
vm.uma.zio_buf_comb_7168.keg.ppera: 2
vm.uma.zio_buf_comb_6144.keg.ppera: 2   <= Broken
vm.uma.zio_buf_comb_5120.keg.ppera: 2
vm.uma.zio_buf_comb_4096.keg.ppera: 1
vm.uma.zio_buf_comb_3584.keg.ppera: 7   <= Hard to free
vm.uma.zio_buf_comb_3072.keg.ppera: 3
vm.uma.zio_buf_comb_2560.keg.ppera: 2
vm.uma.zio_buf_comb_2048.keg.ppera: 1
vm.uma.zio_buf_comb_1536.keg.ppera: 2
vm.uma.zio_buf_comb_1024.keg.ppera: 1
vm.uma.zio_buf_comb_512.keg.ppera: 1

I am now getting such:

vm.uma.zio_buf_comb_16384.keg.ppera: 4
vm.uma.zio_buf_comb_12288.keg.ppera: 3
vm.uma.zio_buf_comb_8192.keg.ppera: 2
vm.uma.zio_buf_comb_6144.keg.ppera: 3   <= Fixed, 2 in 3 pages
vm.uma.zio_buf_comb_4096.keg.ppera: 1
vm.uma.zio_buf_comb_3072.keg.ppera: 3
vm.uma.zio_buf_comb_2048.keg.ppera: 1
vm.uma.zio_buf_comb_1536.keg.ppera: 2
vm.uma.zio_buf_comb_1024.keg.ppera: 1
vm.uma.zio_buf_comb_512.keg.ppera: 1

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[amotin]

Thinking more about it, since most of ZFS memory these days should be in scattered ABD buffers of ARC, it does not make much sense to try and squeeze every byte of transient allocations used by ZIO and relatively small DBUF cache. With many underutilized caches per-CPU overhead on many-CPU systems may end up costing more than that we can save with more granular caches. I am going to change it allocating only half-power of 2 sizes, that should make the code cleaner and life of underlying allocators easier.

[allanjude]

Reviewed-by: Allan Jude <allan@klarasystems.com>

[behlendorf]

Looks good. Using quarter power of 2 allocations always struct me as a bit excessive.