MEDIUM: server: add a new pool-low-conn server setting
The problem with the way idle connections currently work is that it's
easy for a thread to steal all of its siblings' connections, then release
them, then it's done by another one, etc. This happens even more easily
due to scheduling latencies, or merged events inside the same pool loop,
which, when dealing with a fast server responding in sub-millisecond
delays, can really result in one thread being fully at work at a time.
In such a case, we perform a huge amount of takeover() which consumes
CPU and requires quite some locking, sometimes resulting in lower
performance than expected.
In order to fight against this problem, this patch introduces a new server
setting "pool-low-conn", whose purpose is to dictate when it is allowed to
steal connections from a sibling. As long as the number of idle connections
remains at least as high as this value, it is permitted to take over another
connection. When the idle connection count becomes lower, a thread may only
use its own connections or create a new one. By proceeding like this even
with a low number (typically 2*nbthreads), we quickly end up in a situation
where all active threads have a few connections. It then becomes possible
to connect to a server without bothering other threads the vast majority
of the time, while still being able to use these connections when the
number of available FDs becomes low.
We also use this threshold instead of global.nbthread in the connection
release logic, allowing to keep more extra connections if needed.
A test performed with 10000 concurrent HTTP/1 connections, 16 threads
and 210 servers with 1 millisecond of server response time showed the
following numbers:
haproxy 2.1.7: 185000 requests per second
haproxy 2.2: 314000 requests per second
haproxy 2.2 lowconn 32: 352000 requests per second
The takeover rate goes down from 300k/s to 13k/s. The difference is
further amplified as the response time shrinks.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 2aed84e..78439aa 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -13404,6 +13404,19 @@
Actions are disabled by default
+pool-low-conn <max>
+ Set a low threshold on the number of idling connections for a server, below
+ which a thread will not try to steal a connection from another thread. This
+ can be useful to improve CPU usage patterns in scenarios involving many very
+ fast servers, in order to ensure all threads will keep a few idle connections
+ all the time instead of letting them accumulate over one thread and migrating
+ them from thread to thread. Typical values of twice the number of threads
+ seem to show very good performance already with sub-millisecond response
+ times. The default is zero, indicating that any idle connection can be used
+ at any time. It is the recommended setting for normal use. This only applies
+ to connections that can be shared according to the same principles as those
+ applying to "http-reuse".
+
pool-max-conn <max>
Set the maximum number of idling connections for a server. -1 means unlimited
connections, 0 means no idle connections. The default is -1. When idle