Yesterday I wrote:
> When we checked read performance on the untarred tree, we immediately saw
> mixed results. Re-tarring the kernel tree is faster than Ext4, but
directory listing is
> slower by a multiple. So we need to analyze and fix ls without
breaking the good tar
> and untar behavior. The question is, is it worth another delay before
putting Tux3
> patches up for review?
Hirofumi would not let me slink cowardly away from that open question.
We noticed that Tux3 does slightly more than one seek per directory,
which is entirely reasonable. But Ext4 goes way beyond that and does
some special magic to read multiple directories per seek. The only
possible way to do that is, pack directories together so there are many
per track. A bit of sleuthing confirmed that this is indeed the case,
and apparently comes from a patch posted by Ted T'so a few years back:
lwn.net/Articles/319829/
[PATCH, RFC] ext4: New inode/block allocation algorithms for
flex_bg filesystems
That patch was aimed at speeding up fsck and the huge ls speedup appears
to have gone unnoticed.
Thus inspired, Hirofumi whipped up a prototype patch to allocate new
directories first, per delta. Result: Tux3 went from 400% slower to 25%
faster than Ext4, for "ls -R" of the kernel source. Even better, tar and
untar performance stayed about the same with Tux3 topping the untar test
at 20% faster and tar at 350%. (The lopsided tar result looks like a
performance bug for Ext4.)
This optimization only applies to spinning disk. It is pretty hard to
think of a reason why packing directories together would benefit flash.
Maybe, directories that are written together are more likely to be
updated together? But it doesn't hurt flash either, and is another data
point to support our theory that optimizing for spinning disk also
optimizes for flash. We are still waiting for the first counterexample
to show up.
There are a few reasons why Tux3 has an edge for the case exercised by
the kernel source loads:
* Defer everything
Tux3 takes the idea of delayed allocation much further and delays nearly
everything. Directory updates and inode number selection are the only
exceptions. (In future we will attempt to defer the namespace updates as
well.)
* Front back separation
Besides enabling defer-everything, this simplifies locking and reduces
contention a lot, for both read and update. For now, a naive locking
strategy serves us well. Eventually we will multithread the backend,
which will help with high processor core counts, once we get there.
* Big deltas
Under heavy update load, Tux3 deltas grow as big as cache will allow, so
per-delta layout algorithms have a big data window available to optimize
over. With our current strategy, we observe an effect similar to Ext4
flex_bg, where directories and other metadata tend to self-organize
along delta boundaries, with beneficial performance effects. We might
control this behavior more explicitly in future.
* More inodes per inode table block.
Tux3 stores about 57 inodes per block while Ext4 typically has 16 or
less. Multiple inodes per block already resembles a kind of inode table
readahead. Without this, there would be two seeks per directory even
with directories packed together.
Anyway, I don't think we need to hang our heads in shame for performance
reasons at this point, even though plenty of major optimization issues
still remain on the list. For example, you can embarras Tux3 just by
running a benchmark with 10,000 files per directory. The answer to that
one is Shardmap, which needs a couple of months to bring up and solves a
problem that does not come up on your home server or phone. Not a reason
to get sidetracked again..
Regards,
Daniel
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