Hi, On 07/22/2010 08:31 PM, Alvaro Herrera wrote:
FWIW I don't think you should be thinking in "replacing imessages with SLRU". I rather think you should be thinking in how can you implement the imessages API on top of SLRU.
Well, I'm rather comparing SLRU with the dynamic allocator. So far I'm unconvinced that SLRU would be a better base for imessages than a dynamic allocator. (And I'm arguing that SLRU should use a dynamic allocator underneath).
So as far as the coordinator and background worker are concerned, there wouldn't be any difference -- they keep using the same API they are using today.
Agreed, the imessages API to the upper layer doesn't need to care about the underlying stuff.
Also let me repeat my earlier comment about imessages being more similar to multixact than to notify. The content of each multixact entry is just an arbitrary amount of bytes. If imessages are numbered from a monotonically increasing sequence,
Well, there's absolutely no need to serialize imessages. So they don't currently carry any such number. And opposed to multixact entries, they are clearly directed at exactly one single consumer. Every consumer has its own receive queue. Sending messages concurrently to different recipients may happen completely parallelized, without any (b)locking in between.
The dynamic allocator is the only part of the chain which might need to do some locking to protect the shared resource (memory) against concurrent access. Note, however, that wamalloc (as any modern dynamic allocator) is parallelized to some extent, i.e. concurrent malloc/free calls don't necessarily need to block each other.
it should be possible to use a very similar technique, and perhaps you should be able to reduce locking requirements as well (write messages with only a shared lock, after you've determined and reserved the area you're going to write).
Writing to the message is currently (i.e. imessages-on-dynshmem) done without *any* kind of lock held. So that would rather increase locking requirements and lower parallelism, I fear.
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