Re: The usefulness of application logging

James Kanze <>
23 Jul 2006 20:55:56 -0400
Yechezkel Mett wrote:

kanze wrote:

I've still not found a good solution for this when there is a
requirement to be able to change the configuration on the fly
in a multithreaded environment. You certainly don't want a
lock in the if (although at least under Solaris, it isn't
that expensive), so you've got to find some way of suspending
all threads other than the one doing the modification, AND
ensuring that all of the other threads correctly synchronize
their memory after the suspention. The only thing I've found
so far is for the updating thread to post a request in a well
known place, and all other threads to periodically poll and
suspend themselves if the request is present. The updating
thread then waits until all threads are suspended, does the
update, and then signals the threads to continue. While this
is fairly easy to do using conditions in Posix, it results in
a lot of coupling; in particular, the updating thread must
know how many other threads there are, and the other threads
must explicitly collaborate.

The question is how timely it needs to be. Does it matter if
not all threads change configuration immediately? You could
have a thread-local variable caching the current logging state
and update it in each thread (using a lock or similar) when

It's an idea. If thread local variables can be accessed as
quickly as other variables, it is probably a good idea. I'm not
sure that this is always the case, however. The basic code for
each log statement resolves to something along the lines of:
     if ( table[ N ] != NULL ) ...
where N is a constant. So the runtime cost when logging at
level N is turned of is very, very low. If putting table into
thread local memory ups the cost here, it might not be

James Kanze
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