The write paths in storage_proxy pass replica sets as
std::unordered_set<gms::inet_address>. This is a complex type, with
N+1 allocations for N members, so we change it to a small_vector (via
inet_address_vector_replica_set) which requires just one allocation, and
even zero when up to three replicas are used.
This change is more nuanced than the corresponding change to the read path
abe3d7d7 ("Merge 'storage_proxy: use small_vector for vectors of
inet_address' from Avi Kivity"), for two reasons:
- there is a quadratic algorithm in
abstract_write_response_handler::response(): it searches for a replica
and erases it. Since this happens for every replica, it happens N^2/2
times.
- replica sets for writes always include all datacenters, while reads
usually involve just one datacenter.
So, a write to a keyspace that has 5 datacenters will invoke 15*(15-1)/2
=105 compares.
We could remove this by sending the index of the replica in the replica
set to the replica and ask it to include the index in the response, but
I think that this is unnecessary. Those 105 compares need to be only
105/15 = 7 times cheaper than the corresponding unordered_set operation,
which they surely will. Handling a response after a cross-datacenter round
trip surely involves L3 cache misses, and a small_vector reduces these
to a minimum compared to an unordered_set with its bucket table, linked
list walking and managent, and table rehashing.
Tests using perf_simple_query --write --smp 1 --operations-per-shard 1000000
--task-quota-ms show two allocations removed (as expected) and a nice
reduction in instructions executed.
before: median 204842.54 tps ( 54.2 allocs/op, 13.2 tasks/op, 49890 insns/op)
after: median 206077.65 tps ( 52.2 allocs/op, 13.2 tasks/op, 49138 insns/op)
Closes#8847
4d70f3baee