High load in touch thread
-------------------------
Key: CORE-5401
URL: http://tracker.firebirdsql.org/browse/CORE-5401
Project: Firebird Core
Issue Type: Bug
Affects Versions: 2.5.6
Reporter: Alexander Peshkov
A few days ago I posted on Firebird Support "Firebird Recurring Hourly
Load".
The gist of the post is that we noticed that every hour all Firebird
processes would wake up for a second or two causing high load, context
switches etc - This using Firebird 2.5.x on Centos 6.x/7.x.
I've look into this a little bit more and notice that the issue seems to lie
in the trace touch thread - specifically TouchThreadFunc in
jrd/trace/TraceConfigStorage.cpp.
(Please note that I'm not a c++ developer, so there's a good chance that I'm
missing something)
Each thread calls sem_timedwait [while (!semaphore->tryEnter(delay))] with
the next touch time in seconds. The thread then blocks till seconds after
epoch + current milliseconds.
When it returns it compares to the touch time:
if (!m_base->touch_time || m_base->touch_time < now)
Since touch_time is an integer value, current seconds will be equal to it
for up to a full second. This results in a delay of zero being calculated
and sem_trywait being called repeatedly for up to a second per thread until
current seconds > touch_time.
To resolve shouldn't the comparison check for equality:
if (!m_base->touch_time || m_base->touch_time <= now)
I'm also curious as to why each thread is required to wake at the exact same
time - would it not reduce contention if each thread blocked for a full
hour?
Finally, for my own curiosity, what is the reason for touching the trace
file hourly?
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