Converting size to usize is what jemalloc has been done by ceiling
size to the closest size class. However, this causes lots of memory
wastes with HPA enabled. This commit changes how usize is calculated so
that the gap between two contiguous usize is no larger than a page.
Specifically, this commit includes the following changes:
1. Adding a build-time config option (--enable-limit-usize-gap) and a
runtime one (limit_usize_gap) to guard the changes.
When build-time
config is enabled, some minor CPU overhead is expected because usize
will be stored and accessed apart from index. When runtime option is
also enabled (it can only be enabled with the build-time config
enabled). a new usize calculation approach wil be employed. This new
calculation will ceil size to the closest multiple of PAGE for all sizes
larger than USIZE_GROW_SLOW_THRESHOLD instead of using the size classes.
Note when the build-time config is enabled, the runtime option is
default on.
2. Prepare tcache for size to grow by PAGE over GROUP*PAGE.
To prepare for the upcoming changes where size class grows by PAGE when
larger than NGROUP * PAGE, disable the tcache when it is larger than 2 *
NGROUP * PAGE. The threshold for tcache is set higher to prevent perf
regression as much as possible while usizes between NGROUP * PAGE and 2 *
NGROUP * PAGE happen to grow by PAGE.
3. Prepare pac and hpa psset for size to grow by PAGE over GROUP*PAGE
For PAC, to avoid having too many bins, arena bins still have the same
layout. This means some extra search is needed for a page-level request that
is not aligned with the orginal size class: it should also search the heap
before the current index since the previous heap might also be able to
have some allocations satisfying it. The same changes apply to HPA's
psset.
This search relies on the enumeration of the heap because not all allocs in
the previous heap are guaranteed to satisfy the request. To balance the
memory and CPU overhead, we currently enumerate at most a fixed number
of nodes before concluding none can satisfy the request during an
enumeration.
4. Add bytes counter to arena large stats.
To prepare for the upcoming usize changes, stats collected by
multiplying alive allocations and the bin size is no longer accurate.
Thus, add separate counters to record the bytes malloced and dalloced.
5. Change structs use when freeing to avoid using index2size for large sizes.
- Change the definition of emap_alloc_ctx_t
- Change the read of both from edata_t.
- Change the assignment and usage of emap_alloc_ctx_t.
- Change other callsites of index2size.
Note for the changes in the data structure, i.e., emap_alloc_ctx_t,
will be used when the build-time config (--enable-limit-usize-gap) is
enabled but they will store the same value as index2size(szind) if the
runtime option (opt_limit_usize_gap) is not enabled.
6. Adapt hpa to the usize changes.
Change the settings in sec to limit is usage for sizes larger than
USIZE_GROW_SLOW_THRESHOLD and modify corresponding tests.
7. Modify usize calculation and corresponding tests.
Change the sz_s2u_compute. Note sz_index2size is not always safe now
while sz_size2index still works as expected.
it's within the huge page size. These requests do not concern internal
fragmentation with huge pages, since the entire range is expected to be
accessed.
Default SEC max_alloc option value was 32k, disabling SEC for platforms with
lg-page=16. This change enables SEC for all platforms, making minimum max_alloc
value equal to PAGE.
Due to a bug in sec initialization, the number of cached size classes
was equal to 198. The bug caused the creation of more than a hundred of
unused bins, although it didn't affect the caching logic.
Currently used only for guarding purposes, the hint is used to determine
if the allocation is supposed to be frequently reused. For example, it
might urge the allocator to ensure the allocation is cached.
As the code evolves, some code paths that have previously assigned
deferred_work_generated may cease being reached. This would leave the value
uninitialized. This change initializes the value for safety.
Adding guarded extents, which are regular extents surrounded by guard pages
(mprotected). To reduce syscalls, small guarded extents are cached as a
separate eset in ecache, and decay through the dirty / muzzy / retained pipeline
as usual.
This change allows every allocator conforming to PAI communicate that it
deferred some work for the future. Without it if a background thread goes into
indefinite sleep, there is no way to notify it about upcoming deferred work.
This change pulls the SEC options into a struct, which simplifies their handling
across various modules (e.g. PA needs to forward on SEC options from the
malloc_conf string, but it doesn't really need to know their names). While
we're here, make some of the fixed constants configurable, and unify naming from
the configuration options to the internals.
