jemalloc hooks clarifications

[Jakob Buchgraber]

Hello Jason,

I am playing around with the memory management hooks introduced in version
4.
So I wrote a delegate for the default chunk hooks, that additionally report
to
stdout what's happening [1].

The test program allocates 1GB of memory and immediately frees it.
It then tries to allocate 4MB and 8MB. The output is as follows

ALLOC: new_addr 0, size 1073741824, alignment 2097152, zero 1, commit 1,
arena_ind 0, ret 0x7f2f52a00000
DALLOC: chunk 0x7f2f52a00000, size 1073741824, committed 1, arena_ind 0
DECOMMIT: chunk 0x7f2f52a00000, size 1073741824, offset 0, length
1073741824, arena_ind 0
PURGE: chunk 0x7f2f52a00000, size 1073741824, offset 0, length 1073741824,
arena_ind 0
FREED
ALLOC: new_addr 0, size 4194304, alignment 2097152, zero 1, commit 1,
arena_ind 0, ret 0x7f2f52a00000
ALLOC: new_addr 0, size 8388608, alignment 2097152, zero 1, commit 1,
arena_ind 0, ret 0x7f2f52e00000

Given that the 1GB has not been deallocated, but purged I would expect
the last two ALLOCations not to have happened. Instead I would expect
the virtual memory from the 1GB allocation before to be reused?

I am running jemalloc 4.0.4 on Linux.

Also, on an unrelated note, is it generally safe to trigger purging for
arena A
from within an allocation chunk hook of arena B, with A <> B?
The reason why am asking this question is that I would generally want to
run with purging disabled on all arenas, but if some threshold of committed
memory is surpassed I would like to enable purging for some arenas.
Does this sound feasible?

Thanks,
Jakob

[1] https://gist.github.com/buchgr/243b0aa9a2abeda2ac39
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