Re: boot-time slowdown for measure_migration_cost

[Ingo Molnar]

* Bjorn Helgaas <bjorn.helgaas@xxxxxx> wrote:

> The boot-time migration cost auto-tuning stuff seems to have been 
> merged to Linus' tree since 2.6.15.  On little one- or two-processor 
> systems, the time required to measure the migration costs isn't very 
> noticeable, but by the time we get to even a four-processor ia64 box, 
> it adds about 30 seconds to the boot time, which seems like a lot.
> 
> Is that expected?  Is the information we get really worth that much?  
> Could the measurement be done at run-time instead?  Is there a smaller 
> hammer we could use, e.g., flushing just the buffer rather than the 
> *entire* cache? Did we just implement sched_cacheflush() incorrectly 
> for ia64?
> 
> Only ia64, x86, and x86_64 currently have a non-empty 
> sched_cacheflush(), and the x86* ones contain only "wbinvd()". So I 
> suspect that only ia64 sees this slowdown.  But I would guess that 
> other arches will implement it in the future.

the main cost comes from accessing the test-buffer when the buffer size 
gets above the real cachesize. There are a coupe of ways to improve 
that:

- double-check that max_cache_size gets set up correctly on your 
  architecture - the code searches from ~64K to 2*max_cache_size.

- take the values that are auto-detected and use the migration_cost= 
  boot parameter - see Documentation/kernel-parameters.txt:

        migration_cost=
                        [KNL,SMP] debug: override scheduler migration costs
                        Format: <level-1-usecs>,<level-2-usecs>,...
                        This debugging option can be used to override the
                        default scheduler migration cost matrix. The numbers
                        are indexed by 'CPU domain distance'.
                        E.g. migration_cost=1000,2000,3000 on an SMT NUMA
                        box will set up an intra-core migration cost of
                        1 msec, an inter-core migration cost of 2 msecs,
                        and an inter-node migration cost of 3 msecs.

  (a distribution could do this automatically as well in the installer, 
  i've constructed the bootup printout to be in the format that is 
  needed for migration_cost. I have not tested this too extensively 
  though, so double-check the result via an additional 
  migration_debug=2 printout as well! Let me know if you find any bugs 
  here.)

  via this solution you will get zero overhead on subsequent bootups.

- in kernel/sched.c, decrease ITERATIONS from 2 to 1. This will make the 
  measurement more noisy though.

- in kernel/sched.c, change this line:

                size = size * 20 / 19;

  to:

                size = size * 10 / 9;

  this will probably halve the cost - against at the expense of 
  accuracy and statistical stability.

	Ingo
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