(draft) perf: fast-path execution for simple read-only commands (+24% throughput)

[rainsupreme]

Summary

Add a fast execution path in processCommand() that bypasses call() for eligible read-only commands, delivering +24% throughput on ARM Graviton 3 (GET 16B, io-threads=9, P=10).

What it does

For commands that are CMD_READONLY | CMD_FAST (GET, HGET, MGET, EXISTS, TTL, etc.) when no features requiring the full call() ceremony are active, execute the command handler directly with minimal overhead:

• 63/64 commands: cmd->proc(c) + calls++ + stat_numcommands++ + commandlog check. No timing, no zmalloc, no afterCommand.
• 1/64 commands (sampled): Full timing pair, latency histogram, zmalloc peak check, afterCommand cleanup. Microseconds extrapolated ×64 for accurate usec_per_call in INFO.

Why it's ~24% faster

Analysis: Why ~24% faster

CPU flamegraph profiling on the main thread reveals that for a simple
GET, the per-command ceremony in call() costs nearly as much as the actual
work (hash lookup + value serialization + reply building):

Component	% of main-thread time	Skipped on 63/64 cmds?
ustime() timing pair (14ns on ARM)	3.4%	✓
zmalloc_used_memory()	4.7%	✓
afterCommand() + pending flush	3-5%	✓
call() flag save/restore + propagation checks	5-8%	✓
moduleFireCommandResultEvent dispatch	2.5%	✓ (all commands)
updateCommandLatencyHistogram	~1%	✓
Actual command execution (cmd->proc)	~45%	✗ always runs

For O(1) reads like GET, the per-request instruction budget is ~3,200 instructions. The fast path eliminates ~700-800 instructions of ceremony per non-sampled command, which at 2.5M rps and IPC 2.87 directly translates to the observed +24% throughput.

Eligibility conditions

All must be true (falls through to unchanged call() otherwise):

• CMD_READONLY | CMD_FAST flags set
• No module command-result listeners (commandResultSuccessListeners == 0)
• No MONITOR clients connected
• Client-side caching tracking not active
• Top-level execution (execution_nesting == 0)

Covers ~80-95% of commands in typical caching workloads.

What's preserved

• INFO commandstats: calls exact, usec_per_call statistically accurate (1/64 sampling, ~40K samples/sec at 2.5M rps)
• Latency histogram: real duration samples at 1/64 rate
• Command log: always checked (large-reply threshold works on every command; slow-execution threshold uses duration=0 on non-sampled commands — O(1) reads never trigger it anyway)
• stat_numcommands: exact
• Peak memory tracking: sampled 1/64

What's skipped

• moduleFireCommandResultEvent (no-op when no listeners)
• replicationFeedMonitors (no-op when no monitors)
• trackingRememberKeys (disabled when not tracking)
• valkey_commands_trace (LTTng, no-op in default builds)
• Propagation logic (read-only commands never propagate)
• Debug argv assertions
• Per-command timing on 63/64 commands (14ns × 2 saved per command)
• zmalloc_used_memory on 63/64 commands
• afterCommand on 63/64 commands

Benchmark results (ARM Graviton 3, c7g.metal)

Build	RPS	Δ
unstable (baseline)	2,474,204	—
This PR	3,074,640	+24.3%

Config: io-threads=9, P=10, GET 16B real data (pre-populated), 5 reps × 300s, pinned cores 0-8 server / 9-17 client.

Testing

• Unit tests pass with --config io-threads 9 (commandlog, io-threads, keyspace, multi, expire)
• 20M mixed-op stress test (50% GET + 50% SET, io-threads=9, P=10): 0 corrupted values, 0 errors
• Falls through to unchanged call() for any non-eligible command — zero risk to write paths, transactions, modules, or any non-default configuration

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