Re: [PATCH bpf-next v4 0/2] bpf: htab: Reduce memory use of hash maps

[Mykyta Yatsenko]

On 8/13/26 12:19 AM, 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.
> 
> 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%
> 

T.J. are you still interested landing this? Maybe respin the series?
Alexei was away back when you sent this.

> 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 (<= word size), comparing keys only
> requires a single instruction. Currently the 4 byte hash value (8 byte
> aligned and padded) 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 (struct htab_node), 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
> 
> ---
> Changes in v4:
> Removed inline from new functions per BPF CI (netdev/source_inline).
> 
> From Mykyta Yatsenko:
> Factor out duplicate lookup_elem code into __lookup_elem_raw.
> Use offsetof instead of sizeof for key_offset assignments in
> htab_map_alloc (patch 1).
> Eliminate branching and htab_elem casting in htab_elem_hash /
> htab_elem_set_hash.
> 
> Changes in v3:
> From Sashiko on torn reads/writes:
> Use a local unsigned long and READ_ONCE / WRITE_ONCE instead of memcmp /
> memcpy for atomic key comparisons for hashless elements.
> 
> Changes in v2:
> Make maximum key_size for !has_hash depend on word size for atomicity
> on 32-bit.
> 
> From Mykyta Yatsenko:
> Put the htab_elem* common initial sequence in its own struct (htab_node)
> and reuse it across all element types that share it. Eliminate
> associated BUILD_BUG_ON additions.
> Replace both the hash and key fields with data[].
> Store has_hash in struct bpf_htab, and avoid per-element reads of it.# Please edit the description for the branch
> 
> 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                          | 418 ++++++++++++------
>  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, 289 insertions(+), 146 deletions(-)
> 
> 
> base-commit: cfce77b63375dac81d53f2f85593c548415206b7