GC overhead (Re: GCC 3.3, GCC 3.4)

[Matt Austern]

On Monday, February 3, 2003, at 02:30  PM, Geoff Keating wrote:
>> Matt Austern at Apple did similar experiment by using ggc placeholder.
>> (same interface as ggc, no collection, allocate memory using mmap in
>> chunks and
>> satisfy memory allocation requests from these chunks etc..).
>>
>> He got around ~12% speedup for sample c++ source.
>
> To be precise, Matt found that the speedup of using the ggc
> placeholder was about equivalent to the speedup of using ggc-page but
> disabling garbage collection.  This contradicts the results that Tim
> Josling got above.

Yes.  That's why I haven't been pushing to have a mode in which
gc is entirely removed: my measurements suggest that the most
important way in which gc introduces overhead is the most obvious
one: the collection phase itself takes time.

(Insert usual disclaimers about a single measurement not necessarily
being representative, independent replication of results being
important, the possibility of subtle effects on overall design of
the compiler, etc.)

			--Matt