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Re: gcc 3.3 garbage collector defaults
- From: Andi Kleen <ak at suse dot de>
- To: Richard dot Earnshaw at arm dot com
- Cc: Andi Kleen <ak at suse dot de>, Matt Austern <austern at apple dot com>,Zack Weinberg <zack at codesourcery dot com>,Ziemowit Laski <zlaski at apple dot com>, Neil Booth <neil at daikokuya dot co dot uk>,Benjamin Kosnik <bkoz at redhat dot com>, gcc at gcc dot gnu dot org,libstdc++ at gcc dot gnu dot org
- Date: Wed, 29 Jan 2003 13:20:46 +0100
- Subject: Re: gcc 3.3 garbage collector defaults
- References: <20030128220730.GA9662@wotan.suse.de> <200301291144.h0TBi7d14346@pc960.cambridge.arm.com>
I'm not sure I completely understand your reasoning.
On Wed, Jan 29, 2003 at 11:44:04AM +0000, Richard Earnshaw wrote:
> Note that this has much higher transient use of memory, particularly if we
> allocate blocks of memory for trivial purposes, only to discard the result
> very quickly; this is often the case with RTL, where we allocate some RTL
> in order to try something, only to discover that it is not valid and throw
> it away. Further, since B is now dead memory all the effort to bring it
> into the cache has been wasted and we must now bring new memory into the
you mean bringing it into the cache for gc purposes?
Except if you have lots of small objects smaller than a cache line
and your allocator doesn't fully initialize it the working set in
cache should be the same.
If it does memset and your memset is clever enough to use
a write combining write on i386 or dcbz on ppc or similar it will
not even have any fetch traffic on initializing
(the gc does memset a new object when allocating, right?)
> On a sufficiently large compilation (sufficient for the GC to kick in
> several times) the *total* memory used by the compiler will probably be
> about the same for both approaches, but the transient memory usage
> patterns will remain very different, since the GC approach is terrible for
> its short-term reuse of dead memory.
[assuming your objects fit without too much waste into cache lines]
Caches do not really care about how much memory you use overall,
just how much memory you touch in a given time.
When you allocate a lot more memory overall then the caches will be
stressed, because the operating system clearing newly allocated
pages needs to put them into cache too. This overhead should be fairly
limited, if you don't allocate too excessively.
-Andi