Re: PREEMPT_RT and I-PIPE: the numbers, take 3

[Ingo Molnar <mingo@elte.hu>]

* Kristian Benoit <kbenoit@opersys.com> wrote:

> This is the 3rd run of our tests.

i'm still having problems reproducing your numbers, even the 'plain' 
ones. I cannot even get the same ballpark figures, on 3 separate 
systems. To pick one number:

> "plain" run:
> 
> Measurements   |   Vanilla   |  preemp_rt     |
> ---------------+-------------+----------------+
> mmap           |     660us   | 2867us (+334%) |

i was unable to reproduce this level of lat_mmap degradation. I do 
indeed see a slowdown [*], but nowhere near the 4.3x slowdown measured 
here. I have tried the very lmbench version you used (2.0.4) on 3 
different systems (Athlon64 2GHz, Celeron 466MHz, Xeon 2.4GHz - the last 
one should be pretty similar to your 2.8GHz Xeon testbox) and neither 
showed this level of slowdown.

i couldnt figure out which precise options were used by your test, 
because i only found the summary lmbench page of one of the older tests 
- so i did my lat_mmap testing with various sizes: 10MB, 30MB, 70MB, 
150MB, 200MB, 500MB. (My best guess would be that since your target box 
has 512MB of RAM, lmbench will pick an mmap-file size of 144 MB. Or if 
it's the 256MB box, lmbench will pick roughly 70 MB. I covered those 
likely sizes too.) Neither size showed this level of slowdown.

so my tentative conclusion would be that the -RT kernel is still 
misconfigured somehow. Did you have HIGHMEM64 and HIGHPTE enabled 
perhaps? Those i suggested to be turned off in one of my first mails to 
you, it is something that will cause bad performance under PREEMPT_RT.  
(Highmem64 is unwarranted for an embedded test anyway - it's only needed 
to support more than 4 GB of RAM.) Could you send me the test 3 .config 
you used on the -RT kernel?

	Ingo

[*] fixed in -50-36 and later PREEMPT_RT kernels