On Wed, 17 Aug 2005, Linus Torvalds wrote:

On Wed, 17 Aug 2005, Christoph Lameter wrote:

quoted

We are trading 2x (spinlock(page_table_lock), 
spin_unlock(page_table_lock)) against one atomic inc.

Bzzt. Thank you for playing.

Spinunlock is free on x86 and x86-64, since it's a plain normal store. The 
x86 memory ordering semantics take care of the rest.

The same is basically true for ia64 (there is an additional memory barrier 
since ordering must be enforced at that point)

In other words, one uncontended spinlock/unlock pair is pretty much
_exactly_ the same cost as one single atomic operation, and there is no 
win.

We have no problems if the lock are not contended. Then we just reduce the 
overhead by eliminating one semaphore instruction.

If they are contended then spinlock/unlock serializes the page 
fault handler.

Numbers:

Unpatched:

 Gb Rep Threads   User      System     Wall flt/cpu/s fault/wsec
 16   3    1    0.757s     62.772s  63.052s 49515.393  49522.112
 16   3    2    0.674s     68.268s  36.048s 45627.693  86230.400
 16   3    4    0.649s     66.478s  20.061s 46861.663 152603.033
 16   3    8    0.666s    179.611s  25.096s 17449.372 121159.869
 16   3   16    0.721s    545.957s  38.015s  5754.251  82456.599
 16   3   32    1.327s   1718.947s  59.083s  1828.620  52573.584
 16   3   64    3.181s   5260.674s  93.047s   597.609  33651.618
 16   3  128   15.966s  19392.742s 163.090s   162.078  19192.626
 16   3  256   18.214s   9994.286s  84.077s   314.180  37105.725
 16   3  512   13.866s   5023.788s  42.063s   624.443  73776.955

Page fault patches

 Gb Rep Threads   User      System     Wall flt/cpu/s fault/wsec
  4   3    1    0.153s     12.314s  12.047s 63077.153  63065.474
  4   3    2    0.155s     10.430s   5.074s 74290.224 136812.728
  4   3    4    0.157s      9.377s   2.095s 82477.064 266154.766
  4   3    8    0.164s     10.348s   2.002s 74807.804 388714.846
  4   3   16    0.250s     20.687s   2.017s 37560.913 360858.481
  4   3   32    0.568s     43.941s   2.034s 17668.743 335334.362
  4   3   64    2.954s     95.528s   2.066s  7985.502 294723.687
  4   3  128   15.449s    259.534s   3.053s  2859.924 222310.352
  4   3  256   16.108s    137.784s   2.019s  5110.263 357984.896
  4   3  512   14.083s     82.377s   1.049s  8152.851 527180.088

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