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derp/derpserver: use hashtriemap for peer lookup

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Replace the process-global Server.mu lookup in the packet send hot path
with a global hashtriemap mirror of local clientSet entries. The
authoritative clients map remains guarded by Server.mu; clientsAtomic is
only a lock-free fast path for active local clients.

Misses, stale inactive client sets, duplicate accounting, and mesh
forwarding still fall back to lookupDestUncached. This avoids taking
Server.mu for the common local active-client send path, at the cost of
adding one global concurrent map that mirrors Server.clients for local
peers.

The benchmark uses four destination peers. The before run sets
TS_DEBUG_DERP_DISABLE_PEER_HASHTRIE=true to force the old mutex lookup
path; the after run uses the hashtrie fast path.

    goos: linux
    goarch: amd64
    pkg: tailscale.com/derp/derpserver
    cpu: Intel(R) Xeon(R) 6975P-C
                          │    before     │                after                │
                          │    sec/op     │   sec/op     vs base                │
    LookupDestHashTrie-16   176.050n ± 1%   1.904n ± 6%  -98.92% (p=0.000 n=10)

                          │   before   │             after              │
                          │    B/op    │    B/op     vs base            │
    LookupDestHashTrie-16   0.000 ± 0%   0.000 ± 0%  ~ (p=1.000 n=10) ¹
    ¹ all samples are equal

                          │   before   │             after              │
                          │ allocs/op  │ allocs/op   vs base            │
    LookupDestHashTrie-16   0.000 ± 0%   0.000 ± 0%  ~ (p=1.000 n=10) ¹
    ¹ all samples are equal

Updates #3560 (very indirectly, historically)
Updates #19713 (as an alternative to that PR)

Change-Id: Ifb72e5c9854ad00e938cd24c6ab9c27312f297e8
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Brad Fitzpatrick
2026-05-12 20:39:20 +00:00
committed by Brad Fitzpatrick
parent 72578de033
commit 6b729795c3
8 changed files with 214 additions and 18 deletions
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go.mod
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@@ -41,6 +41,7 @@ require (
github.com/go-json-experiment/json v0.0.0-20250813024750-ebf49471dced
github.com/go-logr/zapr v1.3.0
github.com/go-ole/go-ole v1.3.0
github.com/go4org/hashtriemap v0.0.0-20251130024219-545ba229f689
github.com/go4org/plan9netshell v0.0.0-20250324183649-788daa080737
github.com/godbus/dbus/v5 v5.1.1-0.20230522191255-76236955d466
github.com/gokrazy/breakglass v0.0.0-20251229072214-9dbc0478d486