## Proposed Changes
While investigating memory usage I noticed that the malloc metrics were going negative once they passed 2GiB. This is because the underlying `mallinfo` function returns a `i32`, and we were casting it straight to an `i64`, preserving the sign.
The long-term fix will be to move to `mallinfo2`, but it's still not yet widely available.
## Issue Addressed
NA
## Proposed Changes
I've noticed some of the SigP Prater nodes struggling on v1.4.0-rc.0. I suspect this is due to the changes in #2296. Specifically, the trade-off which lowered the memory footprint whilst increasing runtime on some functions.
Presently, this PR is documenting my testing on Prater.
## Additional Info
NA
## Issue Addressed
NA
## Proposed Changes
Modify the configuration of [GNU malloc](https://www.gnu.org/software/libc/manual/html_node/The-GNU-Allocator.html) to reduce memory footprint.
- Set `M_ARENA_MAX` to 4.
- This reduces memory fragmentation at the cost of contention between threads.
- Set `M_MMAP_THRESHOLD` to 2mb
- This means that any allocation >= 2mb is allocated via an anonymous mmap, instead of on the heap/arena. This reduces memory fragmentation since we don't need to keep growing the heap to find big contiguous slabs of free memory.
- ~~Run `malloc_trim` every 60 seconds.~~
- ~~This shaves unused memory from the top of the heap, preventing the heap from constantly growing.~~
- Removed, see: https://github.com/sigp/lighthouse/pull/2299#issuecomment-825322646
*Note: this only provides memory savings on the Linux (glibc) platform.*
## Additional Info
I'm going to close#2288 in favor of this for the following reasons:
- I've managed to get the memory footprint *smaller* here than with jemalloc.
- This PR seems to be less of a dramatic change than bringing in the jemalloc dep.
- The changes in this PR are strictly runtime changes, so we can create CLI flags which disable them completely. Since this change is wide-reaching and complex, it's nice to have an easy "escape hatch" if there are undesired consequences.
## TODO
- [x] Allow configuration via CLI flags
- [x] Test on Mac
- [x] Test on RasPi.
- [x] Determine if GNU malloc is present?
- I'm not quite sure how to detect for glibc.. This issue suggests we can't really: https://github.com/rust-lang/rust/issues/33244
- [x] Make a clear argument regarding the affect of this on CPU utilization.
- [x] Test with higher `M_ARENA_MAX` values.
- [x] Test with longer trim intervals
- [x] Add some stats about memory savings
- [x] Remove `malloc_trim` calls & code
