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>>>>> "Marius" == Marius Vollmer <mvo@zagadka.ping.de> writes:
Doh, forgot to send this to the list :)
Marius> [ compared to just a function call when making write
Marius> barriers explicit. ]
Marius> But the assumption is that the page faults will be quite
Marius> rare, no? MOst mutations occur in the youngest
Marius> generation, and the older generations are static so that
Marius> they don't need to be treated at every gc turn.
One of the problems with this line of thinking is that actual data is
lacking. So: as for the page fault itself, Ben Zorn has measurements
suggesting that the true cost of a protection fault may be close to
10,000 cycles in 1990. (Jones & Lin, _Garbage Collection_, p213), when
Urs Holzle has come up with a way to do write barriers in only two
additional SPARC instructions (p173 in same).
Even if Zorn's measurements are an order of magnitude off, it's still
ridiculous compared to the alternative.
Marius> We would incur the function call overhead at every
Marius> *potential* mutation of a older generation, not only at
Marius> the *actual* mutations.
Yes, but this overhead is two instructions compared to thousands.
--
Graham Hughes <ghughes@cs.ucsb.edu>
PGP Fingerprint: 36 15 AD 83 6D 2F D8 DE EC 87 86 8A A2 79 E7 E6