emqx / mria / 1185

57%

release-1-1: 57%

Pull #222

github

perf: raise the default middleman threshold to 100 messages

Since b857b2a a write operation runs in a helper process when the
caller's message queue has `max_mql' messages or more, 10 by default.
The helper protects mnesia transactions, whose commit stage does
receives that scan the whole queue. For a dirty write of a table with a
local copy there is nothing to protect: it never waits for a message,
and the helper only costs a spawn, a monitor and a wait for the helper
to be scheduled, which on a loaded node is a run-queue round trip per
write.

EMQX runs the route writes of its v3 routing schema in the broker pool
workers, whose queues often hold 10 or more messages under a subscribe
load. At about 10,000 new routes per second per node, one write in ten
took the helper detour. With 100 the detour is left for the long queues
it was made for. The `max_mql' application environment still overrides
the default.

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