[asterisk-dev] [Code Review] Use a shuffling algorithm to find unused IAX2 call numbers

Sean Bright reviewboard at asterisk.org
Thu Feb 14 09:51:13 CST 2013

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(Updated Feb. 14, 2013, 9:51 a.m.)

Review request for Asterisk Developers and rmudgett.

Changes
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* When extracting a call number, don't explicitly zero out the entry at
the beginning of our 'used' list (updated documentation and code)

* Changed 'passed' to 'past.'

* Removed comment about modulo bias.

Summary
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While adding red-black tree containers to astobj2 in r376575, Richard pointed out the way chan_iax2 finds unused call numbers will prevent ao2_container integrity checks at runtime.

This patch removes the ao2_container and instead uses fixed sized arrays and a modified Fisher-Yates-Durstenfeld shuffle to maintain the call number list.  This involves treating the lower indexes of the call number array as the available numbers and the higher indexes as the used ones.  As new call numbers are requested, we choose a random call number from the lower indices of the array, and swap it with the call number that is currently at the end of the available list.  The swapped value becomes the new start to the used list.  Here's an example to illustrate, pretending that there are only 10 call numbers instead of 2^15:

Starting state:

Numbers:   [0][1][2][3][4][5][6][7][8][9]
Available: 10

Choose a random index, pull out the call number at that index, and swap it with the value at the end of our range, and decrement our available numbers:

Chosen:    6
Numbers:   [0][1][2][3][4][5][9][7][8] | [6]
Available: 9

Choose a random index, pull out the call number at that index, and swap it with the value at the end of our range, and decrement our available numbers:

Chosen:    3
Numbers:   [0][1][2][8][4][5][9][7] | [3][6]
Available: 8

And so on.  In the example above, we put the call number that we are extracting at the end of the list, but in reality we don't really care.  The numbers after the end of the usable range are effectively "checked out" and we don't track them - we simply append the returned number to the list of available numbers and increment:

Return:    6
Numbers:   [0][1][2][8][4][5][9][7][6] | [6]
Available: 9

While 3 is outstanding.  In short, the numbers after the "pivot" are irrelevant and we never look at them again, we simply overwrite them with returned numbers.

I haven't done performance testing but I assume this is a bit faster than the current implementation, although the locking behavior is similar.

Diffs (updated)
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/trunk/channels/chan_iax2.c 381401

Diff: https://reviewboard.asterisk.org/r/2288/diff

Testing
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Basic testing generating trunked and non-trunked calls between Asterisk servers.  The call number assignment behavior appears to match the ao2_container approach.

Thanks,

Sean

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