The new macro PLOCK_DISABLE_EBO may be defined to disable exponential
backoff. This can be useful to more easily spot functions that cause
contention. In this case the CPU will be spent inside the functions
themselves instead of the pl_wait_unlock_{long,int}() functions, making
them easier to spot using "perf top" even if that causes a significant
degradation of the thread scalability.
The plock code hasn't been been updated since 2017 and didn't benefit
from the exponential back-off improvements that were added in 2018.
Simply updating the file shows a massive performance gain on large
thread count (>=48) with dequeuing going from 113k RPS to 300k RPS and
round robin from 229k RPS to 1020k RPS. It was about time to update.
In addition, some recent improvements to the code will be useful with
thread groups.
An interesting improvement concerns EPYC CPUs. This one alone increased
fairness and was sufficient to avoid crashes in process_srv_queue() there,
when hammering two servers with maxconn 200 under 1k connections.
As Ilya reported in issue #998, gcc 11 complains about misleading code
indentation which is in fact caused by dead assignments to zero after
a loop which stops on zero. Let's clean both of these.
[ plock commit 4c53fd3a0b2b1892817cebd0db012a52f4087850 ]
Pieter Baauw reported a build issue affecting haproxy after plock was
included. It happens that expressions of the form :
if ((const) ? (expr1) : (expr2))
do_something()
always produce code for both expr1 and expr2 on Clang when building
without optimization. The resulting asm code is even funny, basically
doing :
mov reg, 1
cmp reg, 1
...
This causes our sizeof() tests to fail to build because we purposely
dereference a fake function that reports the location and nature of the
inconsistency, but this fake function appears in the object code despite
all conditions being there to avoid it.
However the compiler is still smart enough to optimize away code doing
if (const)
do_something()
So we simply repeat the condition before do_something(), and the dummy
function is not referenced anymore unless really required.
[ plock commit b155d5c762fb9a9793911881f80e61faa6b0e889 ]
Local variables "l", "i" and "ret" were renamed "__pl_l", "__pl_i" and
"__pl_r" respectively, to limit the risk of conflicts with existing
variables in application code.
