MINOR: tasks: refine the default run queue depth
Since a lot of internal callbacks were turned to tasklets, the runqueue
depth had not been readjusted from the default 200 which was initially
used to favor batched processing. But nowadays it appears too large
already based on the following tests conducted on a 8c16t machine with
a simple config involving "balance leastconn" and one server. The setup
always involved the two threads of a same CPU core except for 1 thread,
and the client was running over 1000 concurrent H1 connections. The
number of requests per second is reported for each (runqueue-depth,
nbthread) couple:
rq\thr| 1 2 4 8 16
------+------------------------------
32| 120k 159k 276k 477k 698k
40| 122k 160k 276k 478k 722k
48| 121k 159k 274k 482k 720k
64| 121k 160k 274k 469k 710k
200| 114k 150k 247k 415k 613k <-- default
It's possible to save up to about 18% performance by lowering the
default value to 40. One possible explanation to this is that checking
I/Os more frequently allows to flush buffers faster and to smooth the
I/O wait time over multiple operations instead of alternating phases
of processing, waiting for locks and waiting for new I/Os.
The total round trip time also fell from 1.62ms to 1.40ms on average,
among which at least 0.5ms is attributed to the testing tools since
this is the minimum attainable on the loopback.
After some observation it would be nice to backport this to 2.3 and
2.2 which observe similar improvements, since some users have already
observed some perf regressions between 1.6 and 2.2.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 450d831..65aabd8 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -2493,10 +2493,12 @@
tune.runqueue-depth <number>
Sets the maximum amount of task that can be processed at once when running
- tasks. The default value is 200. Increasing it may incur latency when
- dealing with I/Os, making it too small can incur extra overhead. When
- experimenting with much larger values, it may be useful to also enable
- tune.sched.low-latency to limit the maximum latency to the lowest possible.
+ tasks. The default value is 40 which tends to show the highest request rates
+ and lowest latencies. Increasing it may incur latency when dealing with I/Os,
+ making it too small can incur extra overhead. When experimenting with much
+ larger values, it may be useful to also enable tune.sched.low-latency and
+ possibly tune.fd.edge-triggered to limit the maximum latency to the lowest
+ possible.
tune.sched.low-latency { on | off }
Enables ('on') or disables ('off') the low-latency task scheduler. By default