Ruby Garbage Collection Heap Slot size

So, ruby enterprise documentation states that all the values in the GC settings are defined in slots: http://www.rubyenterpriseedition.com/documentation.html#_garbage_collector_performance_tuning

(e.g. RUBY_HEAP_MIN_SLOTS)

We fine-tuned our app's min slot size and increment for the best performance by trial and error (we have enough machines to get a good idea how different values affect the number of malloc calls and Full开发者_运维技巧 GCs).

But something has been bugging me for a while: How big is 1 slot in bytes?

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From Ruby source:

 *  sizeof(RVALUE) is
 *  20 if 32-bit, double is 4-byte aligned
 *  24 if 32-bit, double is 8-byte aligned
 *  40 if 64-bit

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$ rvm use ruby-1.9.2-p136
$ gdb ruby
(gdb) p sizeof(RVALUE)
$1 = 40

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The default in 1.9 is 8K

http://svn.ruby-lang.org/repos/ruby/trunk/gc.c (search for HEAP_SIZE)

Note well that whenever it runs out of space and needs to reallocate, in 1.9 it allocates exponentially more heaps.

In 1.8 it would allocate bigger and bigger heaps.

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After diggin' through the code:

1 slot is a size of sizeof(struct RVALUE), which depends on the machine.