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edolstra merged 1 commit into from
Jul 15, 2026
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nix search: Reduce evaluator worker starvation #564

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9 changes: 7 additions & 2 deletions src/libexpr/parallel-eval.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -134,10 +134,15 @@ std::vector<std::future<void>> Executor::spawn(WorkItems && items)
for (auto & item : items) {
std::promise<void> promise;
futures.push_back(promise.get_future());
[[gnu::tls_model("initial-exec")]] static thread_local std::random_device rd;
/* Note: this uses a cheap PRNG rather than std::random_device,
since the latter costs hundreds of cycles per call (RDRAND or
/dev/urandom), which adds up when spawning many work items. The
key only needs to spread items of the same priority around the
queue, not be cryptographically random. */
[[gnu::tls_model("initial-exec")]] static thread_local std::mt19937_64 rng{std::random_device{}()};
[[gnu::tls_model("initial-exec")]] static thread_local std::uniform_int_distribution<uint64_t> dist(
0, 1ULL << 48);
auto key = (uint64_t(item.second) << 48) | dist(rd);
auto key = (uint64_t(item.second) << 48) | dist(rng);
Comment on lines +137 to +145

@coderabbitai coderabbitai Bot Jul 14, 2026

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🎯 Functional Correctness | 🟡 Minor | ⚡ Quick win

🧩 Analysis chain

🏁 Script executed:

#!/usr/bin/env bash
set -euo pipefail

! rg -n '0,\s*1ULL << 48\)' src/libexpr/parallel-eval.cc

Repository: DeterminateSystems/nix-src

Length of output: 200

🏁 Script executed:

#!/usr/bin/env bash
set -euo pipefail

# Show the relevant section with line numbers.
sed -n '120,160p' src/libexpr/parallel-eval.cc

printf '\n---\n'

# Find the type/usage of the queued item and key comparisons.
rg -n 'item\.second|key =|priority|uniform_int_distribution|mt19937_64|queue' src/libexpr/parallel-eval.cc

Repository: DeterminateSystems/nix-src

Length of output: 2531

🌐 Web query:

std::uniform_int_distribution inclusive upper bound cppreference

💡 Result:

In C++, the std::uniform_int_distribution template produces random integers that are uniformly distributed on the closed interval [a, b] [1][2]. This means that both the minimum value (a) and the maximum value (b) are inclusive [1][3]. When constructing the distribution, the second parameter b defaults to the maximum representable value of the integer type (std::numeric_limits::max), which is also included in the potential output range [4][2].

Citations:

🏁 Script executed:

#!/usr/bin/env bash
set -euo pipefail

# Locate the relevant type definitions and queue ordering.
rg -n 'WorkItems|Priority|queue|std::priority_queue|std::map|emplace\(key|item\.second' src/libexpr/parallel-eval.cc

printf '\n---\n'

# Show the top of the file where types and state are declared.
sed -n '1,120p' src/libexpr/parallel-eval.cc

Repository: DeterminateSystems/nix-src

Length of output: 4664

🏁 Script executed:

#!/usr/bin/env bash
set -euo pipefail

# Find the declarations used by Executor::spawn and the queue type.
rg -n 'using WorkItems|struct Item|queue|priority|enum class|enum ' src/libexpr/parallel-eval.hh src/libexpr/parallel-eval.cc

printf '\n--- parallel-eval.hh ---\n'
sed -n '1,220p' src/libexpr/parallel-eval.hh

Repository: DeterminateSystems/nix-src

Length of output: 1103

🏁 Script executed:

#!/usr/bin/env bash
set -euo pipefail

# Locate the parallel-eval headers and the WorkItems / queue declarations.
fd -a 'parallel-eval.hh' src
printf '\n---\n'
fd -a 'parallel-eval.cc' src
printf '\n---\n'
rg -n 'using WorkItems|struct Item|priority|queue|std::map|std::priority_queue|enum class' src/libexpr src -g '!**/build/**' || true

Repository: DeterminateSystems/nix-src

Length of output: 50382

🏁 Script executed:

#!/usr/bin/env bash
set -euo pipefail

# Find the actual location of the parallel-eval header and inspect declarations.
fd -a 'parallel-eval.hh' .
printf '\n---\n'
fd -a 'parallel-eval.cc' .
printf '\n---\n'
rg -n 'using WorkItems|struct Item|priority|queue|std::map|std::priority_queue|enum class' src lib -g '!**/build/**' || true

Repository: DeterminateSystems/nix-src

Length of output: 50382

🏁 Script executed:

#!/usr/bin/env bash
set -euo pipefail

# Locate the Executor/State declaration and the queue container type.
rg -n 'struct State|class Executor|queue' src -g '*parallel*' -g '*eval*' -g '!**/build/**'

printf '\n---\n'
sed -n '1,220p' $(rg -l 'struct State|class Executor' src -g '*parallel*' -g '*eval*' | head -n 1)

Repository: DeterminateSystems/nix-src

Length of output: 7519

Keep the random suffix within bits 0–47. std::uniform_int_distribution is inclusive, so 1ULL << 48 can be returned and set bit 48, overlapping the priority field. Use (1ULL << 48) - 1 instead.

🤖 Prompt for AI Agents 
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@src/libexpr/parallel-eval.cc` around lines 137 - 145, Update the thread-local
std::uniform_int_distribution<uint64_t> dist in the key-generation code to use
(1ULL << 48) - 1 as its inclusive upper bound, keeping the random suffix
confined to bits 0–47 and preventing overlap with the priority field shifted by
48.

state->queue.emplace(key, Item{.promise = std::move(promise), .work = std::move(item.first)});
}
}
Expand Down
6 changes: 6 additions & 0 deletions src/nix/search.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -114,6 +114,12 @@ struct CmdSearch : InstallableValueCommand, MixJSON
work,
std::string_view(state->symbols[attr]).find("Packages") != std::string_view::npos ? 0 : 2,
[cursor2, attrPath2, visit]() { visit(*cursor2, attrPath2, false); });
/* Spawn incrementally rather than after the whole
enumeration, so that idle worker threads can start
on the first attributes while we're still
enumerating the rest. */
if (work.size() >= 256)
futures.spawn(std::exchange(work, {}));
}
futures.spawn(std::move(work));
};
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