On Wed, 04 Nov 2009, Tim Blechmann wrote: > >> well, if i understand the rt howto correctly, _no_ disc access is > >> allowed, neither from rt nor from non-rt threads, since it may produce > >> page faults, which introduce latencies ... > > > > I would be supprised if the rt howto states that page-faults in non-rt > > threads is a critical problem - that would not significantly impact RT > > performance - atelast not the worst case - it will (as every other system > > load) impact the average case. so having a non-rt thread reading disk-files > > to a buffer and a rt-thread processing this buffer should be perfectly fine. > > that is what i thought for years ... but according to [1] a page fault > in an rt process freezes the entire process with both rt and non-rt > threads until the page fault is handled ... > I proved to my self only a few days ago that I know nothing about RT_Preempt so at the risk that this is completly wrong - for a simple test I added a reader/writer thread in cyclictest. with SCHED_OTHER or priority below 50 with SCHED_FIFO the impact is as expected with priority above 50 (left all the irq-threads at there usual default) I see no impact at all in the worst case (average - as can be expected goes up a bit compared to an idle system). here are the results of a short run - the effect pops up imediately for SCHED_OTHER or -p 48, the -p 99 run is about 15 minutes. /* SCHED_OTHER */ rtl26:~/rt/rt-tests# ./cyclictest -n 0.71 0.27 0.16 1/128 6907 T: 0 ( 6906) P: 0 I:1000 C: 9383 Min: 2 Act: 52 Avg: 315 Max: 73704 /* priority below any I/O activity in the kernel */ rtl26:~/rt/rt-tests# ./cyclictest -n -p 48 0.44 0.25 0.16 1/128 6911 T: 0 ( 6910) P:48 I:1000 C: 8610 Min: 1 Act: 2 Avg: 322 Max: 73607 /* priority above any I/O activity in the kernel */ rtl26:~/rt/rt-tests.mod# ./cyclictest -n -p 99 1.60 1.59 1.23 1/128 7314 T: 0 ( 7079) P:99 I:1000 C:1067096 Min: 1 Act: 2 Avg: 2 Max: 31 The thread added simply opens a large file (linux-2.6.31.tar.bz2, 62MB), reading 1k at a time, and writing it to a file in /tmp in an while(1) loop - the thread runs with default attr thus PTHREAD_SCOPE_SYSTEM as SCHED_OTHER thread. so maybe someone with more insight could comment on this "good" result. HW: UP AMD Sempron 1.6GHz, NN IDE disk, 256MB RAM SW: Debian 5.0.3 (Default Desktop) kernel 2.6.31.4-rt14 thx! hofrat -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
