Today, a followup to yesterday’s note with some more details on V8’s new
young-generation implementation, minor mark-sweep or MinorMS.

A
caveat again: these observations are just from reading the code; I
haven’t run these past the MinorMS authors yet, so any of these details
might be misunderstandings.

The MinorMS nursery consists of pages, each of which is 256 kB, unless
huge-page mode is on, in which case they are 2 MB. The total default
size of the nursery is 72 MB by default, or 144 MB if pointer
compression is off.

There can be multiple threads allocating into the nursery, but let’s
focus on the main
allocator,
which is used on the main thread. Nursery allocation is bump-pointer,
whether in a MinorMS page or scavenger semi-space. Bump-pointer regions
are called linear allocation buffers, and often abbreviated as Lab
in the source, though the class is
LinearAllocationArea.

If the current bump-pointer region is too small for the current
allocation, the nursery implementation finds another one, or triggers a
collection. For the MinorMS nursery, each page collects the set of
allocatable spans in a free list; if the free-list is non-empty, it pops
off one entry as the current and tries again.

Otherwise, MinorMS needs another page, and specifically a swept page:
a page which has been visited since the last GC, and whose spans of
unused memory have been collected into a free-list. There is a
concurrent sweeping task which should usually run ahead of the mutator,
but if there is no swept page available, the allocator might need to
sweep some. This logic is in
MainAllocator::RefillLabMain.

Finally, if all pages are swept and there’s no Lab big enough for the
current allocation, we trigger collection from the roots. The initial
roots are the remembered set: pointers from old objects to new
objects. Most of the trace happens concurrently with the mutator; when
the nursery utilisation rises over 90%, V8 will kick off concurrent
marking tasks.

Then once the mutator actually runs out of space, it pauses, drains any pending marking work, marks
conservative roots, then drains again. I am not sure whether MinorMS
with conservative stack scanning visits the whole C/C++ stacks or
whether it manages to install some barriers (i.e. “don’t scan deeper
than 5 frames because we collected then, and so all older frames are
older”); dunno. All of this logic is in
MinorMarkSweepCollector::MarkLiveObjects.

Marking traces the object graph, setting object mark bits. It does not
trace pages. However, the MinorMS space promotes in units of pages. So
how to decide what pages to promote? The answer is that sweeping partitions the MinorMS pages into empty,
recycled, aging, and promoted pages.

Empty pages have no surviving
objects, and are very useful because they can be given back to the
operating system if needed or shuffled around elsewhere in the system. If they are re-used for allocation, they do not need to be swept.

Recycled pages have some survivors, but not many; MinorMS keeps the page
around for allocation in the next cycle, because it has enough empty
space. By default, a page is recyclable if it has 50% or more free
space after a minor collection, or 30% after a major collection.
MinorMS also promotes a page eagerly if in the last cycle, we only
managed to allocate into 30% or less of its empty space, probably due to
fragmentation. These pages need to be swept before re-use.

Finally, MinorMS doesn’t let pages be recycled indefinitely:
after 4 minor cycles, a page goes into the aging pool, in which it is
kept unavailable for allocation for one cycle, but is not yet promoted.
This allows any new allocations on that page in the previous cycle age
out and probably die, preventing premature tenuring.

And that’s it. Next time, a note on a way in which generational
collectors can run out of memory. Have a nice weekend, hackfolk!