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https://issues.apache.org/jira/browse/LUCENE-2232?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12828535#action_12828535
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Paul Elschot commented on LUCENE-2232:
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After the update on the profile, the upperbound on practical gain for from the
profile data would be about:
1/3 of total IndexSearcher.search time spent in readDeltaPosition(),
of this, 7% is used for decoding the VInt when using a disk, and 31% when using
SSD.
So the upperbound on the gain is about 2,3% on a disk, and 10% when using SSD.
I'll try and give a rought estimation based on CPU cycles of actual savings.
(Add the usual disclaimers here.)
Assuming delta positions typically take two bytes:
readVInt does 2 getByte, 1 shift, 2 masks + cond. jumps.
readVShortInt does 2 getByte, 1 shift, 1 mask + cond. jump (this patch does not
avoid the shift).
So the difference is 1 mask + cond. jump on 2 getByte, 1 shift, 2 mask + cond.
jump.
When getByte() is inlined, it reduces to an incremented index array read and a
test.
Excluding the cond. jumps, each mask/shift/index/increment should take about
one clock cycle
when no I/O is necessary.
Another assumption: a mispredicted branch takes 6 cycles, a correctly predicted
branch takes 0.
Assuming all branches are predicted correctly, the total #cycles for readVInt
would be 7.
and 1 mask would be saved by the patch, or about 1/7 of time spent in readVInt
(14%).
With one mispredicted branch, readVInt would take 13 cycles, of which 6 would
be saved (just under 50%)
With two mispredicted branches, 19 cycles, of which 12 would be saved. (about
63%).
Overall (search time) on a disk, from 1/3 * 7% * 14% = 0.3 % to 1/3 * 7% * 64%
= 1.4 % would be saved.
On SSD, this would be about 4,5 (=31/7) times as much, 1.3% to 5.8%.
So on a disk, there is probably no loss or gain because of the extra i/o for
the initial positions,
On ssd this patch could help a few percent, depending on how badly the branches
in readVInt are predicted now.
> Use VShort to encode positions
> ------------------------------
>
> Key: LUCENE-2232
> URL: https://issues.apache.org/jira/browse/LUCENE-2232
> Project: Lucene - Java
> Issue Type: Improvement
> Components: Index
> Reporter: Paul Elschot
> Attachments: LUCENE-2232-nonbackwards.patch,
> LUCENE-2232-nonbackwards.patch
>
>
> Improve decoding speed for typical case of two bytes for a delta position at
> the cost of increasing the size of the proximity file.
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