Re: Scheduler fairness problem on 2.6 series (Attn: Nick Piggin and others)

[Peter Williams <pwil3058@bigpond.net.au>]

Peter Williams wrote:
> Peter Williams wrote:
> 
>> Peter Williams wrote:
>>
>>> William Lee Irwin III wrote:
>>>
>>>> On Tue, Aug 10, 2004 at 07:21:43PM -0700, spaminos-ker@yahoo.com wrote:
>>>>
>>>>> I am not very familiar with all the parameters, so I just kept the 
>>>>> defaults
>>>>> Anything else I could try?
>>>>> Nicolas
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> No. It appeared that the SPA bits had sufficient fairness in them to
>>>> pass this test but apparently not quite enough.
>>>>
>>>
>>> The interactive bonus may interfere with fairness (the throughput 
>>> bonus should actually help it for tasks with equal nice) so you could 
>>> try setting max_ia_bonus to zero (and possibly increasing 
>>> max_tpt_bonus). With "eb" mode this should still give good 
>>> interactive response but expect interactive response to suffer a 
>>> little in "pb" mode however renicing the X server to a negative value 
>>> should help.
>>
>>
>>
>> I should also have mentioned that fiddling with the promotion interval 
>> may help.
> 
> 
> Having reread your original e-mail I think that this problem is probably 
>  being caused by the interactive bonus mechanism classifying the httpd 
> server threads as "interactive" threads and giving them a bonus.  But 
> for some reason the daemon is not identified as "interactive" meaning 
> that it gets given a lower priority.  In this situation if there's a 
> large number of httpd threads (even with promotion) it could take quite 
> a while for the daemon to get a look in.  Without promotion total 
> starvation is even a possibility.
> 
> Peter
> PS For both "eb" and "pb" modes, max_io_bonus should be set to zero on 
> servers (where interactive responsiveness isn't an issue).
> PPS For "sc" mode, try setting "interactive" to zero and "compute" to 1.

I've just run your tests on my desktop and with max_ia_bonus at its 
default value I see the "delta = 3" with 20 threads BUT when I set 
max_ia_bonus to zero they stop (in both "eb" and "pb" mode).  So I then 
reran the tests with 60 threads and zero max_ia_bonus and no output was 
generated by your testdelay script in either "eb" or "pb" modes.  I 
didn't try "sc" mode as I have a ZAPHOD kernel loaded (not HYDRA) but 
Con has reported that the problem is absent in his latest patches so 
I'll update the "sc" mode in HYDRA to those patches.

Peter
-- 
Peter Williams                                   pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
  -- Ambrose Bierce