build: sync between benchies (#8919)

Sometimes, the benchmarks fail to start up pageserver in 10s without any
obvious reason. Benchmarks run sequentially on otherwise idle runners.
Try running `sync(2)` after each bench to force a cleaner slate.

Implement this via:
- SYNC_AFTER_EACH_TEST environment variable enabled autouse fixture
- autouse fixture seems to be outermost fixture, so it works as expected
- set SYNC_AFTER_EACH_TEST=true for benchmarks in build_and_test
workflow

Evidence:
https://neon-github-public-dev.s3.amazonaws.com/reports/main/10678984691/index.html#suites/5008d72a1ba3c0d618a030a938fc035c/1210266507534c0f/

---------

Co-authored-by: Alexander Bayandin <alexander@neon.tech>

.github/workflows/build_and_test.yml

@@ -286,6 +286,7 @@ jobs:
           PERF_TEST_RESULT_CONNSTR: "${{ secrets.PERF_TEST_RESULT_CONNSTR }}"
           TEST_RESULT_CONNSTR: "${{ secrets.REGRESS_TEST_RESULT_CONNSTR_NEW }}"
           PAGESERVER_VIRTUAL_FILE_IO_ENGINE: tokio-epoll-uring
+          SYNC_AFTER_EACH_TEST: true
       # XXX: no coverage data handling here, since benchmarks are run on release builds,
       # while coverage is currently collected for the debug ones
 

test_runner/fixtures/compare_fixtures.py

@@ -1,3 +1,5 @@
+import os
+import time
 from abc import ABC, abstractmethod
 from contextlib import _GeneratorContextManager, contextmanager
 
@@ -8,6 +10,7 @@ import pytest
 from _pytest.fixtures import FixtureRequest
 
 from fixtures.benchmark_fixture import MetricReport, NeonBenchmarker
+from fixtures.log_helper import log
 from fixtures.neon_fixtures import (
     NeonEnv,
     PgBin,
@@ -333,3 +336,26 @@ def neon_with_baseline(request: FixtureRequest) -> PgCompare:
     fixture = request.getfixturevalue(request.param)
     assert isinstance(fixture, PgCompare), f"test error: fixture {fixture} is not PgCompare"
     return fixture
+
+
+@pytest.fixture(scope="function", autouse=True)
+def sync_after_each_test():
+    # The fixture calls `sync(2)` after each test if `SYNC_AFTER_EACH_TEST` env var is `true`
+    #
+    # In CI, `SYNC_AFTER_EACH_TEST` is set to `true` only for benchmarks (`test_runner/performance`)
+    # that are run on self-hosted runners because some of these tests are pretty write-heavy
+    # and create issues to start the processes within 10s
+    key = "SYNC_AFTER_EACH_TEST"
+    enabled = os.environ.get(key) == "true"
+
+    yield
+
+    if not enabled:
+        # regress test, or running locally
+        return
+
+    start = time.time()
+    # we only run benches on unices, the method might not exist on windows
+    os.sync()
+    elapsed = time.time() - start
+    log.info(f"called sync after test {elapsed=}")