On 2019-06-15, Inada Naoki wrote:
> Oh, do you mean your branch doesn't have headers in each page?
That's right. Each pool still has a header but pools can be larger
than the page size. Tim's obmalloc-big-pool idea writes something
to the head of each page within a pool. The radix tree doesn't need
that and actually doesn't care about OS page size.
BTW, the current radix tree doesn't even require that pools are
aligned to POOL_SIZE. We probably want to keep pools aligned
because other parts of obmalloc rely on that.
Here is the matchup of the radix tree vs the current
address_in_range() approach.
- nearly the same in terms of performance. It might depend on OS
and workload but based on my testing on Linux, they are very
close. Would be good to do more testing but I think the radix
tree is not going to be faster, only slower.
- radix tree uses a bit more memory overhead. Maybe 1 or 2 MiB on a
64-bit OS. The radix tree uses more as memory use goes up but it
is a small fraction of total used memory. The extra memory use is
the main downside at this point, I think.
- the radix tree doesn't read uninitialized memory. The current
address_in_range() approach has worked very well but is relying on
some assumptions about the OS (how it maps pages into the program
address space). This is the only aspect where the radix tree is
clearly better. I'm not sure this matters enough to offset the
extra memory use.
- IMHO, the radix tree code is a bit simpler than Tim's
obmalloc-big-pool code. That's not a big deal though as long as
the code works and is well commented (which Tim's code is).
My feeling right now is that Tim's obmalloc-big-pool is the best
design at this point. Using 8 KB or 16 KB pools seems to be better
than 4 KB. The extra complexity added by Tim's change is not so
nice. obmalloc is already extremely subtle and obmalloc-big-pool
makes it more so.
Regards,
Neil
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