Re: calling System.gc to suggest garbage collection
Lew wrote:
Young-generation collections don't touch dead objects.
Peter Duniho wrote:
So? It's not the dead objects that are the problem.
So? As you yourself said,
So, as importantly, a collection that only collects the nursery need not
touch all the other longer-lived objects that are tenured.
So, calling GC forces a full collection, so the tenured generation gets hit,
so you lose the benefit of the young-generation collection, so one should not
so force a collection, for that and other reasons so stated by you.
It's relevant because people thinking of calling 'gc()' may believe that the
total number of objects allocated is relevant, and should be aware that it's
generally only the number of live objects that affect GC time.
As for another point you made,
an application may ... have a large number of objects
in the tenured generation(s).
This is an important point. Many of the things people naively do to "help"
GC, such as reuse of a single object throughout a loop instead of reallocation
in each iteration, force promotion of objects to the tenured generation and
actually hurt memory management. They also tend to promote the scope of
references outside the block where they're used, a bad practice. It is
usually better to limit the lifetime of objects so they get dropped during
minor collections, keeping the size of the tenured generation as small as
practicable. This tends to help limit reference scope also.
Another benefit to keeping lifetimes short is that it helps prevent references
from tenured-generation objects into the young generation, another situation
that unnecessarily tenures objects.
--
Lew