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std: make RwLock test less intensive

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This test called `yield` 80,000 times, which is nothing on a system with
little load, but murder on a CI system. macOS' scheduler in particular
doesn't seem to deal with this very well. The `yield` calls also weren't
even necessarily doing what they were meant to: if the optimizer could
figure out that it doesn't clobber some memory, then it could happily
reorder around the `yield`s anyway!

The test has been simplified and made to work better, and the number of
yields have been reduced. The number of overall iterations has also been
reduced, because with the `yield` calls making races very likely, we
don't really need to run too many iterations to be confident that the
implementation is race-free.
This commit is contained in:
mlugg 2025-09-13 19:57:03 +01:00 • committed by Matthew Lugg
parent 7a5d2a196f
commit b866c14328
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1 changed files with 61 additions and 56 deletions
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117
lib/std/Thread/RwLock.zig
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@ -301,81 +301,86 @@ test "concurrent access" {
const num_writers: usize = 2; const num_writers: usize = 2;
const num_readers: usize = 4; const num_readers: usize = 4;
const num_writes: usize = 4096; const num_writes: usize = 1000;
const num_reads: usize = 8192; const num_reads: usize = 2000;
const Runner = struct { const Runner = struct {
const Self = @This(); const Runner = @This();
rwl: RwLock = .{}, rwl: RwLock,
writes: usize = 0, writes: usize,
reads: std.atomic.Value(usize) = std.atomic.Value(usize).init(0), reads: std.atomic.Value(usize),
term1: usize = 0, val_a: usize,
term2: usize = 0, val_b: usize,
term_sum: usize = 0,
fn reader(self: *Self) !void { fn reader(run: *Runner, thread_idx: usize) !void {
while (true) {
self.rwl.lockShared();
defer self.rwl.unlockShared();
if (self.writes >= num_writes or self.reads.load(.unordered) >= num_reads)
break;
try self.check();
_ = self.reads.fetchAdd(1, .monotonic);
}
}
fn writer(self: *Self, thread_idx: usize) !void {
var prng = std.Random.DefaultPrng.init(thread_idx); var prng = std.Random.DefaultPrng.init(thread_idx);
var rnd = prng.random(); const rnd = prng.random();
while (true) { while (true) {
self.rwl.lock(); run.rwl.lockShared();
defer self.rwl.unlock(); defer run.rwl.unlockShared();
if (self.writes >= num_writes) try testing.expect(run.writes <= num_writes);
break; if (run.reads.fetchAdd(1, .monotonic) >= num_reads) break;
try self.check(); // We use `volatile` accesses so that we can make sure the memory is accessed either
// side of a yield, maximising chances of a race.
const a_ptr: *const volatile usize = &run.val_a;
const b_ptr: *const volatile usize = &run.val_b;
const term1 = rnd.int(usize); const old_a = a_ptr.*;
self.term1 = term1; if (rnd.boolean()) try std.Thread.yield();
try std.Thread.yield(); const old_b = b_ptr.*;
try testing.expect(old_a == old_b);
const term2 = rnd.int(usize);
self.term2 = term2;
try std.Thread.yield();
self.term_sum = term1 +% term2;
self.writes += 1;
} }
} }
fn check(self: *const Self) !void { fn writer(run: *Runner, thread_idx: usize) !void {
const term_sum = self.term_sum; var prng = std.Random.DefaultPrng.init(thread_idx);
try std.Thread.yield(); const rnd = prng.random();
while (true) {
run.rwl.lock();
defer run.rwl.unlock();
const term2 = self.term2; try testing.expect(run.writes <= num_writes);
try std.Thread.yield(); if (run.writes == num_writes) break;
const term1 = self.term1; // We use `volatile` accesses so that we can make sure the memory is accessed either
try testing.expectEqual(term_sum, term1 +% term2); // side of a yield, maximising chances of a race.
const a_ptr: *volatile usize = &run.val_a;
const b_ptr: *volatile usize = &run.val_b;
const new_val = rnd.int(usize);
const old_a = a_ptr.*;
a_ptr.* = new_val;
if (rnd.boolean()) try std.Thread.yield();
const old_b = b_ptr.*;
b_ptr.* = new_val;
try testing.expect(old_a == old_b);
run.writes += 1;
}
} }
}; };
var runner = Runner{}; var run: Runner = .{
var threads: [num_writers + num_readers]std.Thread = undefined; .rwl = .{},
.writes = 0,
.reads = .init(0),
.val_a = 0,
.val_b = 0,
};
var write_threads: [num_writers]std.Thread = undefined;
var read_threads: [num_readers]std.Thread = undefined;
for (threads[0..num_writers], 0..) |*t, i| t.* = try std.Thread.spawn(.{}, Runner.writer, .{ &runner, i }); for (&write_threads, 0..) |*t, i| t.* = try .spawn(.{}, Runner.writer, .{ &run, i });
for (threads[num_writers..]) |*t| t.* = try std.Thread.spawn(.{}, Runner.reader, .{&runner}); for (&read_threads, num_writers..) |*t, i| t.* = try .spawn(.{}, Runner.reader, .{ &run, i });
for (threads) |t| t.join(); for (write_threads) |t| t.join();
for (read_threads) |t| t.join();
try testing.expectEqual(num_writes, runner.writes); try testing.expect(run.writes == num_writes);
try testing.expect(run.reads.raw >= num_reads);
//std.debug.print("reads={}\n", .{ runner.reads.load(.unordered)});
} }