Add test for last written lsn cache

Author: knizhnik

test_runner/performance/test_latency.py

@@ -0,0 +1,34 @@
+import os
+import statistics
+import threading
+import time
+import timeit
+from random import randint
+
+import pytest
+from batch_others.test_backpressure import pg_cur
+from fixtures import log_helper
+from fixtures.benchmark_fixture import MetricReport
+from fixtures.compare_fixtures import PgCompare
+from fixtures.neon_fixtures import Postgres
+
+from performance.test_perf_pgbench import get_scales_matrix
+from performance.test_backpressure import record_read_latency
+
+def start_write_workload(pg: Postgres, scale: int = 10):
+    with pg_cur(pg) as cur:
+        cur.execute(
+            f"create table big as select generate_series(1,{scale*100_000})")
+
+@pytest.mark.parametrize("scale", get_scales_matrix(1))
+def test_measure_read_latency_heavy_write_workload(neon_with_baseline: PgCompare, scale: int):
+    env = neon_with_baseline
+    pg = env.pg
+
+    with pg_cur(pg) as cur:
+        cur.execute(f"create table small as select generate_series(1,{scale*100_000})")
+
+    write_thread = threading.Thread(target=start_write_workload, args=(pg, scale*100))
+    write_thread.start()
+
+    record_read_latency(env, write_thread, "SELECT count(*) from small")