Performance improvements in bcachefs-testing
First off, some background on where we're at currently, regarding metadata IO:

 - A userspace process will never block on IO - i.e., wait for a journal write or a btree node write - unnecessarily. Never ever.  The only reason your userspace proccess will end up blocked waiting for a metadata write to complete is either: you asked to (fsync), or resource exhaustion (either the journal filling up, or we don't have enough memory to allocate a new btree node without flushing some other dirty btree node). Not even indirectly - if there's a lock we need to take to handle some syscall (e.g. btree node locks for a create/rename/etc.), then that lock is never held while we're blocking on IO.

This really is bragging rights all on its own - it means our tail latency is significantly better than other filesystems. If you're a programmer who's ever gotten annoyed by vim hanging when you go to save (probably after you've been compiling your code and generating a bunch of dirty pages) - if you're using bcachefs, that never ever happens. The system just plain feels more responsive and faster - something I hadn't noticed until a user complained about it on IRC when he temporarily had to go back to ext4, because I've been using bcachefs on my development laptop for quite awhile now and it's one of the things you don't think about when it's no longer an annoyance...

So: that part is good, but that doesn't mean there aren't things to improve with respect to metadata writes.

Where we really want to be is to not be _issuing_ any metadata writes, unless we need to to avoid resource exhaustion - that is, ideally btree node writes should only happen when triggered by journal reclaim.

In current bcache-dev, this isn't the case: we'll only buffer up to 4k of dirty data in a btree node before writing it out, and since the default btree node size is 256k for a typical filesystem size that's not very much. Most btree node IO is triggered by hitting that 4k limit, not by journal reclaim.

This is a real issue for performance, for a couple reasons:

- If we buffered up more before writing, we'd end up writing less data overall. Most btree node updates are overwriting an existing key (e.g. updating access times on an inode), so if we buffer up more we'd end up writing out only the latest version of that inode, not all the versions that were overwritten since the previous write.

- More importantly: btree node writes are always issued as FUA writes (forced unit access, i.e. with write caching disabled). If your device doesn't support FUA, the kernel has to emulate it with an expensive flush - and we really don't want to be issuing more flushes than we have to.

- Lastly, since our btree nodes are so big if we buffer up more we can issue much larger writes, which is significantly more efficient than issuing lots of little writes. With our large btree nodes, we should be able to flush metadata drastically more efficiently than any existing filesystem, in fact.

So, this is what I was working on for quite awhile.

The first issue relates to how we manage btree nodes: as mentioned, btree nodes are log structured, with multiple sorted sets of keys. In memory, we sort/compact as needed so that we never have more than three different sets of keys: the lookup and iterator code has to search through and maintain pointers into each sorted set of keys, so we don't want to deal with too many. Having multiple sorted sets of keys ends up being a performance win, since the result is that only the newest and smallest is being modified at any given time, and the rest are constant - we can construct lookup tables for the constant sets of keys that are drastically more efficient for lookup, but wouldn't be possible to update without regenerating the entire lookup table.

The reason we were issuing writes when we had 4k of dirty data buffered up is that inserting into a sorted set of keys that's larger than that is too expensive, mainly because of the memmove() - and the code all assumed that all sets of keys except for the last one were written, so to start a new set of keys we had to write the existing one.

So, fixing that by allowing more than one set of keys to be unwritten was straightforward enough, but as is often the case solving one issue creates another.

The new issue was related to whiteouts. The need for whiteouts also arises from the fact that btree nodes are log structured - if we delete a key that has been written, we have to emit and write out a whiteout, so that when we read the btree node back in we know that key was deleted.

We don't need or want whiteouts in the in memory btree node, so as soon as a set of keys has been written we compact it and drop all the whiteouts. But if we're buffering up a lot more dirty keys, then we're also buffering up a lot more whiteouts - and whiteouts slow down the lookup code, which has to skip past them to find the next live key.

The result of just doing the btree node write changes was that fsmark got quite a bit faster - but rm -rfing all the files fsmark created got quite a bit slower. Oops.

The whiteouts optimizations ended up being quite a bit more complicated - there's a couple things going on in those patches.

Firstly, since we're now buffering up a lot more data before writing out btree nodes, it becomes more worthwhile to make sure we only emit a whiteout when actually needed: if we're deleting a key that was never written, we don't need to emit a whiteout - unless that key was itself overwriting a key that had been written, in which case we do. So tracking when we need to emit whiteouts has some subtlety to it.

The other thing the whiteouts patches do is that when we compact whiteouts from a btree node, the whiteouts that we need to write out are separated from the live keys (where they won't affect the lookup code) and kept in a separate area by themselves, until the btree node is written.

There were a fair number of other smaller optimizations related to whiteouts, but those were the two main ideas.

The end result: on the fsmark benchmark I've been running, btree node writes are reduced by about 80%. Journal writes now dominate, so total metadata IO is cut a little less than in half in that benchmark - but since we'll buffer up to 256k per journal write (assuming nothing is doing a sync() or fsync() to force a journal write), performance for metadata heavy, non sync heavy workloads should be significantly improved.

We're still not optimal with respect to btree node writes: writes for new btree nodes (e.g. from a split or merge) are kicked off immediately, even if the journal won't need those keys to be written for quite some time. Fixing that would be quite a bit more difficult though, and (as seen from the 80% reduction) those are a significantly smaller fraction of metadata IO, so we'll leave that for now.

As always, benchmarking and testing is greatly appreciated. These changes were tricky enough that I really want them tested awhile before going into bcache-dev, so please let me know if you test them and what the results were.

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