volt-vmm/docs/benchmark-warm-start.md

Volt vs Firecracker — Warm Start Benchmark

Date: 2025-03-08 Test Host: Intel Xeon Silver 4210R @ 2.40GHz, 20 cores, Linux 6.1.0-42-amd64 (Debian) Kernel: Linux 4.14.174 (vmlinux ELF, 21,441,304 bytes) — identical for both VMMs Volt Version: v0.1.0 (with i8042 + Seccomp + Caps + Landlock) Firecracker Version: v1.6.0 Methodology: Warm start (all binaries and kernel pre-loaded into OS page cache)

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Executive Summary

Test	Volt (warm)	Firecracker (warm)	Delta
Boot to kernel panic (default)	1,356 ms median	1,088 ms median	NF +268ms (+25%)
Boot to kernel panic (no-i8042)	—	296 ms median	—
Boot to userspace	548 ms median	N/A	—

Key findings:

• Warm start times are nearly identical to cold start times — this confirms that disk I/O is not a bottleneck for either VMM
• The ~268ms gap between Volt and Firecracker persists (architectural, not I/O related)
• Both VMMs show excellent consistency in warm start: ≤2.3% spread for Volt, ≤3.3% for Firecracker
• Volt boots to a usable shell in 548ms warm, demonstrating sub-second userspace availability

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1. Warm Boot to Kernel Panic — Side by Side

Both VMMs booting the same kernel with console=ttyS0 reboot=k panic=1 pci=off, no rootfs, 128MB RAM, 1 vCPU. Time measured from process start to "Rebooting in 1 seconds.." appearing in serial output.

Volt (20 iterations)

Run	Time (ms)		Run	Time (ms)
1	1,348		11	1,362
2	1,356		12	1,339
3	1,359		13	1,358
4	1,355		14	1,370
5	1,345		15	1,359
6	1,348		16	1,341
7	1,349		17	1,359
8	1,363		18	1,355
9	1,339		19	1,357
10	1,343		20	1,361

Firecracker (20 iterations)

Run	Time (ms)		Run	Time (ms)
1	1,100		11	1,090
2	1,082		12	1,075
3	1,100		13	1,078
4	1,092		14	1,086
5	1,090		15	1,086
6	1,090		16	1,102
7	1,073		17	1,067
8	1,085		18	1,087
9	1,072		19	1,103
10	1,095		20	1,088

Statistics — Boot to Kernel Panic (default boot args)

Statistic	Volt	Firecracker	Delta
Min	1,339 ms	1,067 ms	+272 ms
Max	1,370 ms	1,103 ms	+267 ms
Mean	1,353.3 ms	1,087.0 ms	+266 ms (+24.5%)
Median	1,355.5 ms	1,087.5 ms	+268 ms (+24.6%)
Stdev	8.8 ms	10.3 ms	NF tighter
P5	1,339 ms	1,067 ms	—
P95	1,363 ms	1,102 ms	—
Spread	31 ms (2.3%)	36 ms (3.3%)	NF more consistent

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2. Firecracker — Boot to Kernel Panic (no-i8042)

With i8042.noaux i8042.nokbd added to boot args, eliminating the ~780ms i8042 probe timeout.

Run	Time (ms)		Run	Time (ms)
1	304		11	289
2	292		12	293
3	311		13	296
4	294		14	307
5	290		15	299
6	297		16	296
7	312		17	301
8	296		18	286
9	293		19	304
10	317		20	283

Statistic	Value
Min	283 ms
Max	317 ms
Mean	298.0 ms
Median	296.0 ms
Stdev	8.9 ms
P5	283 ms
P95	312 ms
Spread	34 ms (11.5%)

Note: Volt emulates the i8042 controller, so it responds to keyboard probes instantly (no timeout). Adding i8042.noaux i8042.nokbd to Volt's boot args wouldn't have the same effect since the probe already completes without delay. The ~268ms gap between Volt (1,356ms) and Firecracker-default (1,088ms) comes from other architectural differences, not i8042 handling.

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3. Volt — Warm Boot to Userspace

Boot to "VOLT VM READY" banner (volt-init shell prompt). Same kernel + 260KB initramfs, 128MB RAM, 1 vCPU.

Run	Time (ms)		Run	Time (ms)
1	560		11	552
2	576		12	556
3	557		13	562
4	557		14	538
5	556		15	544
6	534		16	538
7	538		17	534
8	530		18	549
9	525		19	547
10	552		20	534

Statistic	Value
Min	525 ms
Max	576 ms
Mean	547.0 ms
Median	548.0 ms
Stdev	12.9 ms
P5	525 ms
P95	562 ms
Spread	51 ms (9.3%)

Headline: Volt boots to a usable userspace shell in 548ms (warm). This is faster than either VMM's kernel-only panic time because the initramfs provides a root filesystem, avoiding the slow VFS panic path entirely.

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4. Warm vs Cold Start Comparison

Cold start numbers from benchmark-comparison-updated.md (10 iterations each):

Test	Cold Start (median)	Warm Start (median)	Improvement
NF → kernel panic	1,338 ms	1,356 ms	~0% (within noise)
NF → userspace	548 ms	548 ms	0%
FC → kernel panic	1,127 ms	1,088 ms	−3.5%
FC → panic (no-i8042)	351 ms	296 ms	−15.7%

Analysis

1. Volt cold ≈ warm: The 3.45MB binary and 21MB kernel load so fast from disk that page cache makes no measurable difference. This is excellent — it means Volt has no I/O bottleneck even on cold start.

2. Firecracker improves slightly warm: FC sees a modest 3-16% improvement from warm cache, suggesting slightly more disk sensitivity (possibly from the static-pie binary layout or memory mapping strategy).

3. Firecracker no-i8042 sees biggest warm improvement: The 351ms → 296ms drop suggests that when kernel boot is very fast (~138ms internal), the VMM startup overhead becomes more prominent, and caching helps reduce that overhead.

4. Both are I/O-efficient: Neither VMM is disk-bound in normal operation. The binaries are small enough (3.4-3.5MB) to always be in page cache on any actively-used system.

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5. Boot Time Breakdown

Why Volt with initramfs (548ms) boots faster than without (1,356ms)

This counterintuitive result is explained by the kernel's VFS panic path:

Phase	Without initramfs	With initramfs
VMM init	~85 ms	~85 ms
Kernel early boot	~300 ms	~300 ms
i8042 probe	~0 ms (emulated)	~0 ms (emulated)
VFS mount attempt	Fails → panic path (~950ms)	Succeeds → runs init (~160ms)
Total	~1,356 ms	~548 ms

The kernel panic path includes stack dump, register dump, reboot timer (1 second in panic=1), and serial flush — all adding ~800ms of overhead that doesn't exist when init runs successfully.

VMM Startup: Volt vs Firecracker

Phase	Volt	Firecracker (--no-api)	Notes
Binary load + init	~1 ms	~5 ms	FC larger static binary
KVM setup	0.1 ms	~2 ms	Both minimal
CPUID config	35 ms	~10 ms	NF does 46-entry filtering
Memory allocation	34 ms	~30 ms	Both mmap 128MB
Kernel loading	14 ms	~12 ms	Both load 21MB ELF
Device setup	0.4 ms	~5 ms	FC has more device models
Security hardening	0.9 ms	~2 ms	Both apply seccomp
Total to VM running	~85 ms	~66 ms	FC ~19ms faster startup

The gap is primarily in CPUID configuration: Volt spends 35ms filtering 46 CPUID entries vs Firecracker's ~10ms. This represents the largest optimization opportunity.

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6. Consistency Analysis

VMM	Test	Stdev	CV (%)	Notes
Volt	Kernel panic	8.8 ms	0.65%	Extremely consistent
Volt	Userspace	12.9 ms	2.36%	Slightly more variable (init execution)
Firecracker	Kernel panic	10.3 ms	0.95%	Very consistent
Firecracker	No-i8042	8.9 ms	3.01%	More relative variation at lower absolute

Both VMMs demonstrate excellent determinism in warm start conditions. The coefficient of variation (CV) is under 3% for all tests, with Volt's kernel panic test achieving the tightest distribution at 0.65%.

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7. Methodology

Test Setup

• Same host, same kernel, same conditions for all tests
• 20 iterations per measurement (plus 2-3 warm-up runs discarded)
• All binaries pre-loaded into OS page cache (cat binary > /dev/null)
• Wall-clock timing via date +%s%N (nanosecond precision)
• Named pipe (FIFO) for real-time serial output detection without buffering delays
• Guest config: 1 vCPU, 128 MB RAM
• Boot args: console=ttyS0 reboot=k panic=1 pci=off i8042.noaux (Volt default)
• Boot args: console=ttyS0 reboot=k panic=1 pci=off (Firecracker default)

Firecracker Launch Mode

• Used --no-api --config-file mode (no REST API socket overhead)
• This is the fairest comparison since Volt also uses direct CLI launch
• Previous benchmarks used the API approach which adds ~8ms socket startup overhead

What "Warm Start" Means

1. All binary and kernel files read into page cache before measurement begins
2. 2-3 warm-up iterations run and discarded (warms KVM paths, JIT, etc.)
3. Only subsequent iterations counted
4. This isolates VMM + KVM + kernel performance from disk I/O

Measurement Point

• "Boot to kernel panic": Process start → "Rebooting in 1 seconds.." in serial output
• "Boot to userspace": Process start → "VOLT VM READY" in serial output
• Detection via FIFO pipe (mkfifo) with line-by-line scanning for marker string

Caveats

1. Firecracker v1.6.0 (not v1.14.2 as in previous benchmarks) — version difference may affect timing
2. Volt adds i8042.noaux to boot args by default; Firecracker's config used bare pci=off
3. Both tested without jailer/cgroup isolation for fair comparison
4. FIFO-based timing adds <1ms measurement overhead

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Raw Data

Volt — Kernel Panic (sorted)

1339 1339 1341 1343 1345 1348 1348 1349 1355 1355
1356 1357 1358 1359 1359 1359 1361 1362 1363 1370

Volt — Userspace (sorted)

525 530 534 534 534 538 538 538 544 547
549 552 552 556 556 557 557 560 562 576

Firecracker — Kernel Panic (sorted)

1067 1072 1073 1075 1078 1082 1085 1086 1086 1087
1088 1090 1090 1090 1092 1095 1100 1100 1102 1103

Firecracker — No-i8042 (sorted)

283 286 289 290 292 293 293 294 296 296
296 297 299 301 304 304 307 311 312 317

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Generated by automated warm-start benchmark suite, 2025-03-08 Benchmark script: /tmp/bench-warm2.sh