The goal is to make allocation less likely to fail. With async
reclaimer there is an implicit bound on the amount of memory that can
be allocated between deferring points. This bound is difficult to
enforce though. Sync reclaimer lifts this limitation off.
Also, allocations which could not be satisfied before because of
fragmentation now will have higher chances of succeeding, although
depending on how much memory is fragmented, that could involve
evicting a lot of segments from cache, so we should still avoid them.
Downside of sync reclaiming is that now references into regions may be
invalidated not only across deferring points but at any allocation
site. compaction_lock can be used to pin data, preferably just
temporarily.
This heavily used function shows up in many places in the profile (as part
of other functions), so it's worth optimizing by eliminating the special
case for the standard allocator. Use a statically allocated object instead.
(a non-thread-local object is fine since it has no data members).
Some code may attempt to use it during finalization after "instance"
was destroyed.
Reported by Pekka:
/usr/include/c++/4.9.2/bits/unique_ptr.h:291:14: runtime error:
reference binding to null pointer of type 'struct
standard_allocation_strategy'
./utils/allocation_strategy.hh:105:13: runtime error: reference
binding to null pointer of type 'struct standard_allocation_strategy'
./utils/allocation_strategy.hh:118:35: runtime error: reference
binding to null pointer of type 'struct allocation_strategy'
./utils/managed_bytes.hh:59:45: runtime error: member call on null
pointer of type 'struct allocation_strategy'
./utils/allocation_strategy.hh:82:9: runtime error: member access
within null pointer of type 'struct allocation_strategy'
