Re: [PATCH v2 9/9] reftable/stack: handle locked tables during auto-compaction
From: Karthik Nayak <hidden>
Date: 2024-08-08 12:25:29
Patrick Steinhardt [off-list ref] writes:
When compacting tables, it may happen that we want to compact a set of tables which are already locked by a concurrent process that compacts them. In the case where we wanted to perform a full compaction of all tables it is sensible to bail out in this case, as we cannot fulfill the requested action. But when performing auto-compaction it isn't necessarily in our best interest of us to abort the whole operation. For example, due to the geometric compacting schema that we use, it may be that process A takes a lot of time to compact the bulk of all tables whereas process B appends a bunch of new tables to the stack. B would in this case also notice that it has to compact the tables that process A is compacting already and thus also try to compact the same range, probably including the new tables it has appended. But because those tables are locked already, it will fail and thus abort the complete auto-compaction. The consequence is that the stack will grow longer and longer while A isn't yet done with compaction, which will lead to a growing performance impact. Instead of aborting auto-compaction altogether, let's gracefully handle this situation by instead compacting tables which aren't locked. To do so, instead of locking from the beginning of the slice-to-be-compacted, we start locking tables from the end of the slice. Once we hit the first table that is locked already, we abort. If we succeded to lock two or
s/succeded/succeeded
more tables, then we simply reduce the slice of tables that we're about to compact to those which we managed to lock. This ensures that we can at least make some progress for compaction in said scenario. It also helps in other scenarios, like for example when a process died and left a stale lockfile behind. In such a case we can at least ensure some compaction on a best-effort basis.
Right. This is really well written. [snip]
quoted hunk ↗ jump to hunk
@@ -1052,19 +1062,47 @@ static int stack_compact_range(struct reftable_stack *st, /* * Lock all tables in the user-provided range. This is the slice of our * stack which we'll compact. + * + * Note that we lock tables in reverse order from last to first. The + * intent behind this is to allow a newer process to perform best + * effort compaction of tables that it has added in the case where an + * older process is still busy compacting tables which are preexisting + * from the point of view of the newer process. */ REFTABLE_CALLOC_ARRAY(table_locks, last - first + 1); - for (i = first; i <= last; i++) { - stack_filename(&table_name, st, reader_name(st->readers[i])); + for (i = last + 1; i > first; i--) { + stack_filename(&table_name, st, reader_name(st->readers[i - 1])); err = hold_lock_file_for_update(&table_locks[nlocks], table_name.buf, LOCK_NO_DEREF); if (err < 0) { - if (errno == EEXIST) + /* + * When the table is locked already we may do a + * best-effort compaction and compact only the tables + * that we have managed to lock so far. This of course + * requires that we have been able to lock at least two + * tables, otherwise there would be nothing to compact. + * In that case, we return a lock error to our caller. + */ + if (errno == EEXIST && last - (i - 1) >= 2 && + flags & STACK_COMPACT_RANGE_BEST_EFFORT) { + err = 0; + /* + * The subtraction is to offset the index, the + * addition is to only compact up to the table + * of the preceding iteration. They obviously + * cancel each other out, but that may be + * non-obvious when it was omitted. + */ + first = (i - 1) + 1;
I remember thinking about this and how to fix it, I'm happy how you've done it. It's simple and sufficient. Kudos. [snip] Overall this patch looked great. I have nothing to add, I really like it that we use a flag and this means this would only come into play for auto-compaction.
Attachments
- signature.asc [application/pgp-signature] 690 bytes