On Fri Mar 7, 2025 at 7:36 AM CET, Alexei Starovoitov wrote:
> On Wed, Mar 5, 2025 at 6:33 AM <arthur@xxxxxxxxxxxxxxx> wrote:
> >
> > +struct __trait_hdr {
> > + /* Values are stored ordered by key, immediately after the header.
> > + *
> > + * The size of each value set is stored in the header as two bits:
> > + * - 00: Not set.
> > + * - 01: 2 bytes.
> > + * - 10: 4 bytes.
> > + * - 11: 8 bytes.
>
> ...
>
> > + * - hweight(low) + hweight(high)<<1 is offset.
>
> the comment doesn't match the code
>
> > + */
> > + u64 high;
> > + u64 low;
>
> ...
>
> > +static __always_inline int __trait_total_length(struct __trait_hdr h)
> > +{
> > + return (hweight64(h.low) << 1) + (hweight64(h.high) << 2)
> > + // For size 8, we only get 4+2=6. Add another 2 in.
> > + + (hweight64(h.high & h.low) << 1);
> > +}
>
> This is really cool idea, but 2 byte size doesn't feel that useful.
> How about:
> - 00: Not set.
> - 01: 4 bytes.
> - 10: 8 bytes.
> - 11: 16 bytes.
>
> 4 byte may be useful for ipv4, 16 for ipv6, and 8 is just a good number.
> And compute the same way with 3 popcount with extra +1 to shifts.
I chose the sizes arbitrarily, happy to change them.
16 is also useful for UUIDs, for tracing.
Size 0 could store bools / flags. Keys could be set without a value,
and users could check if the key is set or not.
That replaces single bits of the mark today, for example a
"route locally" key.
That only leaves one other size, maybe 4 for smaller values?
If we want more sizes, we could also:
- Add another u64 word to the header, so we have 3 bits per key. It
uses more room, and we need more popcnts, but most modern x86 CPUs can
do 3 popcnts in parallel so it could be ok.
- Let users set consecutive keys to one big value. Instead of supporting
size 16, we let them set two 8 byte KVs in one trait_set() call and
provide a 16 byte value. Eg:
trait_set_batch(u64 key_from, u64_key_to, size, ...);
It's easy to implement, but it makes the API more complicated.
