Ceki Gulcu wrote:
Joern Huxhorn wrote:
Nope, a discussion like that is really necessary because this whole issue is pretty crucial and will essentially decide how well Logback will perform in environments with >1 CPU-cores. This will become more and more important with more cores added in every new CPU generation. The more cores are present, the more relevant the issue of unfairness will become, since waiting threads will mean idling CPUs.
While open to further investigation and new data, evidence collected indicates that threads in Sun's JDK on multi-core CPU on Linux as well as other platforms can make other threads starve but that is *not* a logback issue. Moreover, it would be unfair to penalize *all* logback users using fair locking which is much slower than synchronization or unfair locks.
I copied your results over from your test for the sake of discussion: java.runtime.version = 1.6.0_11-b03 java.vendor = Sun Microsystems Inc. java.version = 1.6.0_11 os.name = Linux os.version = 2.6.25-gentoo-r6 Sync: 139698775, or 35 nanos per cycle Unfair: 87028802, or 57 nanos per cycle Fair: 4106449, or 1217 nanos per cycle java.runtime.version = jvmxa6460-20081105_25433 java.vendor = IBM Corporation java.version = 1.6.0 os.name = Linux os.version = 2.6.25-gentoo-r6 Sync: 356946629, or 14 nanos per cycle Unfair: 91892449, or 54 nanos per cycle Fair: 7380308, or 677 nanos per cycle So yes, the locking itself is significantly slower. The following numbers are the max amount of wasted time of my example app: 2959018000 synchronized 2078606000 unfair lock 27691000 fair lock The maximum wasted time of all threads is ~100 times smaller in case of the fair lock!! This means that the overall system is running *waaay* smoother... regardless of the additional nanos spent for the fair lock. The main problem is that *every* CPU has to access the appender, degrading multi-core-systems to semi-single-core. Minimizing the amount of time a thread (and therefore one of the other cores) has to wait is crucial.
As a side note, and if my memory serves me well, you gather caller data for every log call which can have significant impact on performance. Are you collecting caller data? Yes, I do, but performance was absolutely sufficient on a single-core system - which is quite ironic. Let's just forget about my appender for the moment. The behavior is similar but less dramatic in case of FileAppender.
Regards, Joern.