Re: Tux3 Report: Untar Unleashed

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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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