The routing table's shards started empty and grew as routes came in,
each growth moving every row of the shard into a table twice the size.
In a profile of the daemon taking in a full table, the growth showed as
up to a seventh of the session's time, and the daemon's resident memory
had the old and the new table of each shard side by side for a moment.
This change adds `RibTable::expect_prefixes`, which reserves room for
that many prefixes spread over the shards, and has the server call it at
startup with the largest `expected_routes` among the configured peers:
the prefixes of several full tables are mostly the same ones. Without
`expected_routes` nothing changes. The `full_table` benchmark does the
same unless `BENCH_PRESIZE=0`.
Measured, the saving in time is within the run-to-run noise: in the
Linux VM one peer's table is in the routing table in 0.81 s instead of
0.89 s, and the kernel is in step at about the same time as before;
three pairs of runs of the benchmark on a Mac average 0.33 s against
0.35 s. The profile's attribution to the growth was mostly sampling skid.
The memory saving is real: 442 MiB resident with one peer's table in
instead of 483 MiB, and 626 MiB instead of 632 MiB with four.
Also: the profiling script printed nothing for the kernel's change
counts because it asked `/metrics` for them; the endpoint is `/metricz`.
With that fixed, a four-peer load shows 1.25 million installs for 1.23
million prefixes: the journal already gives the kernel task only the
latest change of a prefix when it lags, so no further coalescing of
installs is called for.
Documentation: `expected_routes` in the configuration reference, the
neighbor guide and the proto comment say that the table is sized by it;
the testing guide's VM results are brought up to date.
Testing: a table test checks that the room is there and is kept; the
workspace tests pass.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
