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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 succeeded to lock two or 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. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>
Git - fast, scalable, distributed revision control system
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