Re: [PATCH] Hold an 'unsigned long' chunk of the sha1 in obj_hash

Jeff King [off-list ref] writes:

It _might_ still be advantageous to do your patch on top, but I suspect
it will diminish the returns from your patch (since the point of it is
to probe less far down the chain on average).

No, mine makes it slower again.  Apparently the increased size is no
longer worth it.  You'll need a wide window to read this:

Test                                  next              tr/hash-speedup            jk/hash-speedup             both-hash-speedup        
----------------------------------------------------------------------------------------------------------------------------------------
0001.1: rev-list --all                0.66(0.63+0.02)   0.66(0.63+0.03) -0.4%      0.66(0.63+0.03) -0.6%       0.66(0.62+0.03) -0.6%    
0001.2: rev-list --all --objects      4.12(4.05+0.05)   3.81(3.74+0.06) -7.6%***   3.50(3.43+0.05) -15.1%***   3.56(3.49+0.05) -13.7%***
----------------------------------------------------------------------------------------------------------------------------------------

Note that the scripts always generate the percentages and significance
w.r.t. the first column.  Comparing yours with both instead gives

Test                               jk/hash-speedup   both-hash-speedup     
---------------------------------------------------------------------------
0001.1: rev-list --all             0.66(0.63+0.03)   0.66(0.62+0.03) +0.0% 
0001.2: rev-list --all --objects   3.50(3.43+0.05)   3.56(3.49+0.05) +1.6%*
---------------------------------------------------------------------------

which is still significant (in the statistical p=5% sense).

For kicks I also ran some other tests, which generally show that the
speedups are limited to this specific workload:

Test                                  next              tr/hash-speedup          jk/hash-speedup         both-hash-speedup     
-------------------------------------------------------------------------------------------------------------------------------
3201.1: branch --contains             0.76(0.74+0.02)   0.75(0.73+0.02) -1.0%*   0.77(0.74+0.02) +0.7%   0.76(0.73+0.02) -0.7% 
4000.1: log -3000 (baseline)          0.12(0.09+0.02)   0.12(0.10+0.02) +3.2%    0.12(0.10+0.01) +0.0%   0.12(0.10+0.02) +3.2% 
4000.2: log --raw -3000 (tree-only)   0.53(0.47+0.05)   0.52(0.46+0.05) -0.9%    0.53(0.46+0.06) +0.0%   0.52(0.45+0.06) -0.9% 
4000.3: log -p -3000 (Myers)          2.39(2.23+0.14)   2.38(2.23+0.13) -0.4%    2.38(2.23+0.14) -0.4%   2.38(2.23+0.13) -0.5% 
4000.4: log -p -3000 --histogram      2.43(2.28+0.13)   2.43(2.28+0.13) -0.0%    2.43(2.28+0.13) -0.1%   2.43(2.28+0.14) +0.1% 
4000.5: log -p -3000 --patience       2.72(2.57+0.12)   2.74(2.59+0.12) +0.7%    2.72(2.57+0.13) +0.2%   2.74(2.59+0.13) +0.8%.
-------------------------------------------------------------------------------------------------------------------------------

-- 
Thomas Rast
trast@{inf,student}.ethz.ch