reporter: Alexander Peshkov [ alexpeshkoff ] => David Hollings [ dholling ]

assignee: Alexander Peshkov [ alexpeshkoff ]

status: Open [ 1 ] => Resolved [ 5 ]

resolution: Fixed [ 1 ]

Fix Version: 2.5.7 [ 10770 ]

description: 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?

=>

David Hollings posted on Firebird Support "Firebird Recurring Hourly Load".

The gist of the post is that he 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?

status: Resolved [ 5 ] => Resolved [ 5 ]

QA Status: No test => Cannot be tested

status: Resolved [ 5 ] => Closed [ 6 ]