map Lookup allocates when key does not exist

[brycekahle]

Describe the bug

In a hot loop of Lookup where a lot of keys do not exist, the fmt.Errorf at

ebpf/map.go

Line 698 in fbb9ed8

return fmt.Errorf("lookup: %w", wrapMapError(err))

can allocate a decent chunk of memory

How to reproduce

1. Create map
2. Do a bunch of lookups for keys that do not exist
3. Profile allocations

Version information

github.com/cilium/ebpf v0.15.1-0.20240709101333-5976561b28aa

[lmb]

Can you add top output from pprof or similar? I wonder whether most of the cost is in wrapMapError or in fmt.Errorf. We could consider special casing ENOENT. Plus add a test so we don't regress (this sounds familiar!)

[brycekahle]

(pprof) top
Showing nodes accounting for 26991765, 74.69% of 36140090 total
Dropped 227 nodes (cum <= 180700)
Showing top 10 nodes out of 93
      flat  flat%   sum%        cum   cum%
   7618793 21.08% 21.08%    7618793 21.08%  fmt.Errorf

I don't see wrapMapError in my profiles at all.

[brycekahle]

From a flamegraph: