Ruby Memory Allocation
During a recent discussion at work, my ever so sharp coworker James Tucker noted the significant void between user code and low level optimizations.
Micro benchmarks and rubyspec compliance are frequently referenced to compare implementations and quite often cited in random rants as well.What happens at that intermediate, libc, level ?
In this post we’ll explore the ISO C memory allocation / deallocation functions as invoked by the following interpreters :
* ruby-1.8.7-p72 (./configure)
* ruby-1.9.1-p129 (./configure)
* jruby-1.3.1 (binary distribution)
Requirements
* A DTrace enabled OS (OS X Leopard, Solaris etc.)
* One or more interpreters installed
* A local checkout of extlib
I’ve opted to NOT test with the latest 1.9.2 preview – see why
The DTrace PID provider
A set of C level probes is spawned for the lifetime of a given command or existing process.When the process dies, the probe’s is unset.You don’t have to recompile or relink – it just works.See the Sun wiki for further information.
This is very different from static probes that ships with MySQL 6 or a previously released patched tree by Joyent.
Memory Allocation
Regardless of the underlying GC implementation, there’s three ISO C functions that handle memory allocation.
* malloc allocates X bytes of memory
* calloc allocates memory for X objects of Y size each
* realloc shrinks or expands the size of a previously allocated memory region
Each allocation function returns a pointer to the allocated (or reallocated) memory region which can be released with the free function call.
Of note is that most implementations never release memory from the heap back to the underlying OS – it’s assigned to a pool for reallocation to reduce system call (sbrk) overhead of the current process.
Toolbox
Ruby 1.8.7
I happen to use this as my stock 1.8 series interpreter – 1.8.6 should yield similar output.
Ruby 1.9.1
JRuby 1.3.1
Again, I’ve noticed a 1.4 version flying around, but happened to have 1.3.1 installed.
Passing Thoughts
Note the liberal memory allocation overheads on the 1.8 series, but an otherwise overall similar distribution pattern to 1.9.I can’t comment much on the JVM case due to limited Java experience.Anyone care to chime in ?
This is an exercise to test the waters for a potential audience for these experiments.I have a few contexts lined up, with a specific MRI focus, but can get into the bowels of most things.Suggestions and thoughts welcome.
26 Responses to “Ruby Memory Allocation”
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Charles Oliver Nutter on August 9th, 2009
The reason for the peculiar JRuby results is that since the JVM uses a generational, compacting garbage collector, it allocates large sections of memory at once and then partitions them as needed. The vast majority of objects created by the JVM use the heap and the GC. What you’re seeing here, I believe, is the memory allocated by native code outside the heap.
This is also, of course, why the JVM has such fast object allocation rates; each thread gets its own buffer of memory to allocate as it sees fit, and once requested from the system it’s just a matter of biting off pieces. After the initial heap allocation, object allocation is “nearly free”.
JVM engineers can feel free to correct me on any of this, but it gives you a rough idea what’s going on.
admin on August 9th, 2009
Charles,
Appreciate the comment – prompted further investigation and discovered the JVM allocates heap with mmap.
Here’s the output of a barebones 4 line source file, assuming the mmap calls to be on JVM startup.
I tried to intercept brk and sbrk syscalls as well, but assume they’d only be invoked for larger code bases that exceeds the initial heap size at runtime.
- Lourens
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