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ShardedInt provides an int type expvar.Var that supports more efficient writes at high frequencies (one order of magnigude on an M1 Max, much more on NUMA systems). There are two implementations of ShardValue, one that abuses sync.Pool that will work on current public Go versions, and one that takes a dependency on a runtime.TailscaleP function exposed in Tailscale's Go fork. The sync.Pool variant has about 10x the throughput of a single atomic integer on an M1 Max, and the runtime.TailscaleP variant is about 10x faster than the sync.Pool variant. Neither variant have perfect distribution, or perfectly always avoid cross-CPU sharing, as there is no locking or affinity to ensure that the time of yield is on the same core as the time of core biasing, but in the average case the distributions are enough to provide substantially better performance. See golang/go#18802 for a related upstream proposal. Updates tailscale/go#109 Updates tailscale/corp#25450 Signed-off-by: James Tucker <james@tailscale.com>
37 lines
1.1 KiB
Go
37 lines
1.1 KiB
Go
// Copyright (c) Tailscale Inc & AUTHORS
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// SPDX-License-Identifier: BSD-3-Clause
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package syncs
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// TODO(raggi): this implementation is still imperfect as it will still result
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// in cross CPU sharing periodically, we instead really want a per-CPU shard
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// key, but the limitations of calling platform code make reaching for even the
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// getcpu vdso very painful. See https://github.com/golang/go/issues/18802, and
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// hopefully one day we can replace with a primitive that falls out of that
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// work.
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// ShardValue contains a value sharded over a set of shards.
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// In order to be useful, T should be aligned to cache lines.
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// Users must organize that usage in One and All is concurrency safe.
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// The zero value is not safe for use; use [NewShardValue].
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type ShardValue[T any] struct {
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shards []T
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//lint:ignore U1000 unused under tailscale_go builds.
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pool shardValuePool
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}
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// Len returns the number of shards.
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func (sp *ShardValue[T]) Len() int {
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return len(sp.shards)
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}
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// All yields a pointer to the value in each shard.
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func (sp *ShardValue[T]) All(yield func(*T) bool) {
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for i := range sp.shards {
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if !yield(&sp.shards[i]) {
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return
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}
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}
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}
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