Re: [PATCH 0/2] bpf: htab: Reduce memory use of hash maps

[T.J. Mercier]

On Thu, Jul 23, 2026 at 7:11 AM Mykyta Yatsenko
<mykyta.yatsenko5@xxxxxxxxx> wrote:
>
> On 7/22/26 9:37 PM, T.J. Mercier wrote:
> > Memory is expensive and scarce these days. This series reduces the
> > memory use of BPF hash maps by eliminating the per-element overheads
> > below. This saves up to 50% of per-element memory use for standard and
> > PCPU hash maps. The memory use of LRU hash maps is unaffected.
> >
>
> I understand that these savings calculations do not account for the element
> value overhead of per-CPU maps, which make up most of the map memory
> consumption. Realistically we won't see any memory savings for per-CPU maps.

Yes, I did not look at BPF_MAP_TYPE_PERCPU_ARRAY at all. 95% of the
memory used by all of our BPF maps comes from hashmaps affected by
these changes. We only have 7 BPF_MAP_TYPE_PERCPU_ARRAY maps and they
consume only about 12 KiB. (I know a few of those PCPU arrays are just
to avoid 128 / 256 byte BPF stack allocations.)





> > Map Type & Configuration            | Old size | New size | Savings
> > ------------------------------------|----------|----------|--------
> > Standard (key <= 8 B, val <= 8 B)   |   64 B   |   32 B   | 50.0%
> > Per-CPU (prealloc) (key <= 8 B)     |   64 B   |   32 B   | 50.0%
> > Per-CPU (non-prealloc) (key <= 8 B) |   64 B   |   40 B   | 37.5%
> > LRU (Any key/value size)            |    -     |    -     | 00.0%
> >
> > 1) Unused LRU / PCPU fields in standard and PCPU hash maps (patch 1)
> > struct htab_elem is used for all hash map types, and includes fields
> > that are not always used (bpf_lru_node, ptr_to_pptr). For standard
> > (non-LRU, non-PCPU) hash maps the 24 bytes for the bpf_lru_node (union)
> > are entirely overhead and can be eliminated. Non-preallocated PCPU maps
> > only need the 8 byte ptr_to_pptr which is currently unioned with the
> > unneeded 24 byte bpf_lru_node, so 16 bytes of overhead can be
> > eliminated. Preallocated PCPU maps don't need ptr_to_pptr, so 24 bytes
> > of overhead can be saved.
> >
> > 2) Hash caching for small keys (patch 2)
> > For hash maps with small key sizes (<= 8 bytes), comparing keys only
> > requires a single instruction (64 bit), or a few (32 bit). Currently the
> > 4 byte hash value (8 byte aligned) is used for this, but offers no
> > performance advantage in this case and can be eliminated.
> >
> > The implementation splits htab_elem into dedicated structures for the
> > different map types (htab_elem_lru, htab_elem_pcpu, htab_elem) which
> > share a common initial sequence, but contain additional map-type
> > specific fields where necessary. This means the placement of the key for
> > each element varies with the map type, and key_offset is added to
> > bpf_htab for this purpose.
> >
> > While using key_offset and conditional hash checks adds new pointer
> > dereferences and branching during element traversal,
> > run_bench_htab_mem.sh shows no significant performance regression across
> > 10 runs on my 3995WX.
> >
> > Benchmark (all in kops/sec)  | Avg. Before | StDev | Avg. After | StDev
> > -----------------------------|-------------|-------|------------|------
> > prealloc overwrite           | 115.11      | 4.10  | 115.45     | 5.24
> > prealloc batch_add_batch_del | 127.14      | 4.32  | 127.06     | 2.32
> > prealloc add_del_on_diff_cpu | 23.22       | 0.93  | 22.91      | 1.60
> > normal overwrite             | 78.52       | 3.05  | 80.40      | 3.25
> > normal batch_add_batch_del   | 45.37       | 0.69  | 47.71      | 0.66
> > normal add_del_on_diff_cpu   | 12.02       | 0.73  | 12.48      | 0.70
> >
> > T.J. Mercier (2):
> >   bpf: htab: Split htab_elem_lru and htab_elem_pcpu off of htab_elem
> >   bpf: htab: Reduce elem_size by 8 bytes for small key sizes
> >
> >  kernel/bpf/hashtab.c                          | 339 ++++++++++++------
> >  kernel/bpf/map_in_map.c                       |  13 +
> >  kernel/bpf/map_in_map.h                       |   2 +
> >  .../selftests/bpf/progs/map_ptr_kern.c        |   2 +-
> >  4 files changed, 252 insertions(+), 104 deletions(-)
> >
> >
> > base-commit: cfce77b63375dac81d53f2f85593c548415206b7
>