[RFC/Benchmarks] Per-CPU freelists

Suleiman Souhlal ssouhlal at FreeBSD.org
Sun Jan 15 20:43:36 PST 2006


I implemented per-CPU page freelists this weekend, in the hopes that it 
would improve  performance on SMP machines, as it should save a spinlock 
acquisition in vm_page_alloc(), in most cases (except when 
VM_ALLOC_INTERRUPT is set, and when the current cpu's free list is 
empty), and reduce contention. However, I was only able to test it on 
machine with two CPUs, where it didn't seem to make any difference.

You can set the number of pages that get added to the freelists each 
time it gets refilled in vm.pcpu.refill_num, and the maximum length of 
the freelists in vm.pcpu.max_len. Some stats are viewable in vm.pcpu.stats.

The patch is available at 

I would really appreciate if someone could benchmark/test this on a 
machine with more processors.

Here's the output of ministat(1) for buildkernel:
x refill_num=32, max_len=64
+ refill_num=4, max_len=-1 (effectively disabling the percpu freelists)
|+         +           x  x+ x                       +       +  x         x|
|    |_____________|_______M_MA____________A____________|__________|       |
     N           Min           Max        Median           Avg        Stddev
x   5        171.64        172.09        171.69       171.816    0.21220273
+   5        171.44        171.97        171.67       171.702    0.22928149
No difference proven at 95.0% confidence

-- Suleiman

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