go build -gcflags="-m" ./...

GODEBUG=gctrace=1 ./your-binary

GODEBUG=gctrace=1 ./your-binary 2>&1 | python3 - <<'PY'
import re,sys
pat = re.compile(r'^gc (\d+) @([0-9.]+)s ([0-9.]+)%: ([0-9.]+)\+([0-9.]+)\+([0-9.]+) ms clock, .*?, ([0-9.]+)->([0-9.]+)->([0-9.]+) MB,')
print('cycle,stw_ms,mark_ms,live_mb,gc_cpu_pct')
for line in sys.stdin:
    m = pat.match(line.strip())
    if not m:
        continue
    cycle, t, gc_pct, stw1, mark, stw2, h1, h2, live = m.groups()
    stw = float(stw1) + float(stw2)
    print(f"{cycle},{stw:.2f},{mark},{live},{gc_pct}")
PY

GODEBUG=gctrace=1 ./your-binary 2>&1 | python3 - <<'PY'
import re,sys,math
pat = re.compile(r'^gc (\d+) @([0-9.]+)s ([0-9.]+)%: ([0-9.]+)\+([0-9.]+)\+([0-9.]+) ms clock,')
vals = []
for line in sys.stdin:
    m = pat.match(line.strip())
    if not m:
        continue
    stw1, stw2 = float(m.group(4)), float(m.group(6))
    vals.append(stw1 + stw2)
 
def pct(xs,p):
    if not xs:
        return None
    xs = sorted(xs)
    k = (len(xs)-1) * p
    f, c = math.floor(k), math.ceil(k)
    if f == c:
        return xs[int(k)]
    return xs[f] + (xs[c] - xs[f]) * (k - f)
 
p95 = pct(vals, 0.95)
p99 = pct(vals, 0.99)
print(f"p95_stw_ms={p95:.2f} p99_stw_ms={p99:.2f} samples={len(vals)}")
PY

GODEBUG=gctrace=1 ./your-binary 2>&1 | python3 - <<'PY'
import re,sys
import matplotlib.pyplot as plt
pat = re.compile(r'^gc (\d+) @([0-9.]+)s ([0-9.]+)%: ([0-9.]+)\+([0-9.]+)\+([0-9.]+) ms clock, .*?, ([0-9.]+)->([0-9.]+)->([0-9.]+) MB,')
t, stw = [], []
for line in sys.stdin:
    m = pat.match(line.strip())
    if not m:
        continue
    t.append(float(m.group(2)))
    stw.append(float(m.group(4)) + float(m.group(6)))
plt.plot(t, stw)
plt.xlabel('time (s)')
plt.ylabel('stw (ms)')
plt.title('GC STW over time')
plt.show()
PY

GODEBUG=gctrace=1 ./your-binary 2>&1 | python3 - <<'PY'
import re,sys,math
import matplotlib.pyplot as plt
pat = re.compile(r'^gc (\d+) @([0-9.]+)s ([0-9.]+)%: ([0-9.]+)\+([0-9.]+)\+([0-9.]+) ms clock,')
t, stw = [], []
for line in sys.stdin:
    m = pat.match(line.strip())
    if not m:
        continue
    t.append(float(m.group(2)))
    stw.append(float(m.group(4)) + float(m.group(6)))
 
def pct(xs,p):
    xs = sorted(xs)
    k = (len(xs)-1) * p
    f, c = math.floor(k), math.ceil(k)
    if f == c:
        return xs[int(k)]
    return xs[f] + (xs[c] - xs[f]) * (k - f)
 
p95 = pct(stw, 0.95)
p99 = pct(stw, 0.99)
plt.plot(t, stw, label='stw')
plt.axhline(p95, color='orange', ls='--', label='p95')
plt.axhline(p99, color='red', ls='--', label='p99')
plt.xlabel('time (s)')
plt.ylabel('stw (ms)')
plt.title('GC STW over time')
plt.legend()
plt.show()
PY

GODEBUG=gctrace=1 ./your-binary 2>&1 | python3 - <<'PY'
import re,sys
import matplotlib.pyplot as plt
pat = re.compile(r'^gc (\d+) @([0-9.]+)s ([0-9.]+)%: ([0-9.]+)\+([0-9.]+)\+([0-9.]+) ms clock, .*?, ([0-9.]+)->([0-9.]+)->([0-9.]+) MB,')
t, stw, live, gc = [], [], [], []
for line in sys.stdin:
    m = pat.match(line.strip())
    if not m:
        continue
    t.append(float(m.group(2)))
    stw.append(float(m.group(4)) + float(m.group(6)))
    live.append(float(m.group(9)))
    gc.append(float(m.group(3)))
 
fig, ax1 = plt.subplots()
ax2 = ax1.twinx()
 
l1, = ax1.plot(t, stw, color='C0', label='stw_ms')
l2, = ax2.plot(t, live, color='C1', label='live_mb')
l3, = ax2.plot(t, gc, color='C2', label='gc_cpu_pct')
 
ax1.set_xlabel('time (s)')
ax1.set_ylabel('stw (ms)')
ax2.set_ylabel('live MB / GC CPU %')
ax1.set_title('GC STW + Live Heap + GC CPU')
 
lines = [l1, l2, l3]
ax1.legend(lines, [l.get_label() for l in lines])
plt.show()
PY

GODEBUG=gctrace=1 ./your-binary 2>&1 | python3 - <<'PY'
import re,sys
pat = re.compile(r'^gc (\d+) @([0-9.]+)s ([0-9.]+)%: ([0-9.]+)\+([0-9.]+)\+([0-9.]+) ms clock, .*?, ([0-9.]+)->([0-9.]+)->([0-9.]+) MB,')
print('time_s,stw_ms')
for line in sys.stdin:
    m = pat.match(line.strip())
    if not m:
        continue
    t = m.group(2)
    stw = float(m.group(4)) + float(m.group(6))
    print(f"{t},{stw:.2f}")
PY > gctrace.csv
 
gnuplot -e "set datafile separator ','; set term png size 900,300; set output 'stw.png'; set xlabel 'time (s)'; set ylabel 'stw (ms)'; plot 'gctrace.csv' using 1:2 with lines title 'stw'"

GODEBUG=gctrace=1 ./your-binary 2>&1 | python3 - <<'PY' > gctrace.csv
import re,sys
pat = re.compile(r'^gc (\d+) @([0-9.]+)s ([0-9.]+)%: ([0-9.]+)\+([0-9.]+)\+([0-9.]+) ms clock, .*?, ([0-9.]+)->([0-9.]+)->([0-9.]+) MB,')
print('time_s,stw_ms')
for line in sys.stdin:
    m = pat.match(line.strip())
    if not m:
        continue
    t = m.group(2)
    stw = float(m.group(4)) + float(m.group(6))
    print(f"{t},{stw:.2f}")
PY
 
read P95 P99 <<EOF
$(python3 - <<'PY'
import csv,math
with open('gctrace.csv') as f:
    next(f)
    vals=[float(r.split(',')[1]) for r in f if r.strip()]
vals.sort()
 
def pct(xs,p):
    k=(len(xs)-1)*p
    f=math.floor(k); c=math.ceil(k)
    if f==c:
        return xs[int(k)]
    return xs[f]+(xs[c]-xs[f])*(k-f)
 
print(f"{pct(vals,0.95)} {pct(vals,0.99)}")
PY)
EOF
 
gnuplot -e "set datafile separator ','; set term png size 900,300; set output 'stw.png'; set xlabel 'time (s)'; set ylabel 'stw (ms)'; p95=$P95; p99=$P99; plot 'gctrace.csv' using 1:2 with lines title 'stw', p95 with lines lt 1 lc rgb 'orange' title 'p95', p99 with lines lt 1 lc rgb 'red' title 'p99'"

export GOGC=80
export GOMEMLIMIT=1GiB

export GOMEMLIMIT=768MiB

// alloc-heavy
func buildMany(n int) []string {
  out := make([]string, 0, n)
  for i := 0; i < n; i++ {
    out = append(out, fmt.Sprintf("id=%d", i
  }
  return out
}
 
// lower-alloc
func buildManyBetter(n int) []string {
  out := make([]string, 0, n)
  var b strings.Builder
  for i := 0; i < n; i++ {
    b.Reset()
    b.WriteString("id=")
    b.WriteString(strconv.Itoa(i))
    out = append(out, b.String())
  }
  return out
}