[Bug nptl/11588] pthread condvars are not priority inheritance aware

["dvhltc at us dot ibm dot com" <sourceware-bugzilla@sourceware.org>]

------- Additional Comments From dvhltc at us dot ibm dot com  2010-05-11 18:55 -------
Created an attachment (id=4782)
 --> (http://sourceware.org/bugzilla/attachment.cgi?id=4782&action=view)
pi condvar patch impact on normal path testcase

This test is designed to test the efficiency of the pthread_cond_wait() and
pthread_cond_signal() code paths within glibc. Ulrich has expressed concern
about these patches negatively impacting the non-pi condvar paths. I felt the
changes to the common case (when PI is not involved) were pretty minimal.
Certainly any significant regression there would be unacceptable.

I built three versions of glibc 2.11.1 from git:
1)	  git: unmodified git sources
2)     c_only: pthread_cond*.S files deleted
3) pi_condvar: same as c_only with the pi_condvar patches applied

Comparing #3 against #2 allows us to eliminate any gains #1 would have solely
from the hand written asm. 3 will eventually contain hand written asm, but
until the non-posix API is agreed upon, it doesn't make sense to expend the
effort of writing the asm code in my opinion.

I then ran 10 runs of 10M iterations each at SCHED_FIFO 1 priority on each of
the three glibcs, the results (following) suggest no significant change in the
non PI condvar performance, sitting right at ~270k (avg) cycles/sec for each
glibc.

build-x86_64-2.11.1-git
Cycles/Second: 279831.187500
Cycles/Second: 261911.421875
Cycles/Second: 277664.125000
Cycles/Second: 284847.718750
Cycles/Second: 285067.281250
Cycles/Second: 267918.718750
Cycles/Second: 284785.656250
Cycles/Second: 277402.843750
Cycles/Second: 202379.703125
Cycles/Second: 266421.718750

Min:  202379.703125  us
Max:  285067.28125  us
Avg:  268823.0375  us


build-x86_64-2.11.1-c_only
Cycles/Second: 277931.781250
Cycles/Second: 275614.093750
Cycles/Second: 271194.125000
Cycles/Second: 280155.093750
Cycles/Second: 284708.156250
Cycles/Second: 190936.031250
Cycles/Second: 264253.468750
Cycles/Second: 281354.281250
Cycles/Second: 290366.218750
Cycles/Second: 279962.000000

Min:  190936.03125  us
Max:  290366.21875  us
Avg:  269647.525  us


build-x86_64-2.11.1-pi_condvar
Cycles/Second: 263975.937500
Cycles/Second: 279577.281250
Cycles/Second: 276504.531250
Cycles/Second: 266163.562500
Cycles/Second: 262115.796875
Cycles/Second: 279219.406250
Cycles/Second: 265263.812500
Cycles/Second: 262226.468750
Cycles/Second: 284592.687500
Cycles/Second: 278975.875000

Min:  262115.796875  us
Max:  284592.6875  us
Avg:  271861.535938  us


This is only the cond_signal case, and doesn't account for cond_timedwait or
cond_broadcast, but I wouldn't expect those to experience any additional impact
from this patch. Are there scenarios you can think of that are likely to suffer
greater impact that are not covered by this rather simple test case?

-- 


http://sourceware.org/bugzilla/show_bug.cgi?id=11588

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