Don't write atime file if JAX_COMPILATIION_CACHE_MAX_SIZE == -1

The atime file is only needed to implement the LRU eviction policy,
which is only needed if a max persistence compilation cache size is
set. Writing this file can cause network filesystem performace and
other issues, so only write it if users are opted-in.

CHANGELOG.md

@@ -22,7 +22,7 @@ When releasing, please add the new-release-boilerplate to docs/pallas/CHANGELOG.
     JAX-level dead code elimination (DCE). See {jax-issue}`#25956` for more
     details.
   * Added low-level reduction APIs in {mod}`jax.lax`: {func}`jax.lax.reduce_sum`,
-    {func}`jax.lax.reduce_prod`, {func}`jax.lax.reduce_max`, {func}`jax.lax.reduce_min`, 
+    {func}`jax.lax.reduce_prod`, {func}`jax.lax.reduce_max`, {func}`jax.lax.reduce_min`,
     {func}`jax.lax.reduce_and`, {func}`jax.lax.reduce_or`, and {func}`jax.lax.reduce_xor`.
   * {func}`jax.lax.linalg.qr`, and {func}`jax.scipy.linalg.qr`, now support
     column-pivoting on CPU and GPU. See {jax-issue}`#20282` and
@@ -37,6 +37,12 @@ When releasing, please add the new-release-boilerplate to docs/pallas/CHANGELOG.
     This package may safely be removed if it is present on your machine; JAX now
     uses `libtpu` instead.
 
+* Bug fixes
+  * Persistent compilation cache no longer writes access time file if
+    JAX_COMPILATION_CACHE_MAX_SIZE is unset or set to -1, i.e. if the LRU
+    eviction policy isn't enabled. This should improve performance when using
+    the cache with large-scale network storage.
+
 ## jax 0.5.0 (Jan 17, 2025)
 
 As of this release, JAX now uses

jax/_src/lru_cache.py

@@ -94,7 +94,6 @@ def get(self, key: str) -> bytes | None:
       raise ValueError("key cannot be empty")
 
     cache_path = self.path / f"{key}{_CACHE_SUFFIX}"
-    atime_path = self.path / f"{key}{_ATIME_SUFFIX}"
 
     if self.eviction_enabled:
       self.lock.acquire(timeout=self.lock_timeout_secs)
@@ -108,8 +107,10 @@ def get(self, key: str) -> bytes | None:
 
       val = cache_path.read_bytes()
 
-      timestamp = time.time_ns().to_bytes(8, "little")
-      atime_path.write_bytes(timestamp)
+      if self.eviction_enabled:
+        timestamp = time.time_ns().to_bytes(8, "little")
+        atime_path = self.path / f"{key}{_ATIME_SUFFIX}"
+        atime_path.write_bytes(timestamp)
 
       return val
 
@@ -138,7 +139,6 @@ def put(self, key: str, val: bytes) -> None:
       return
 
     cache_path = self.path / f"{key}{_CACHE_SUFFIX}"
-    atime_path = self.path / f"{key}{_ATIME_SUFFIX}"
 
     if self.eviction_enabled:
       self.lock.acquire(timeout=self.lock_timeout_secs)
@@ -151,8 +151,10 @@ def put(self, key: str, val: bytes) -> None:
 
       cache_path.write_bytes(val)
 
-      timestamp = time.time_ns().to_bytes(8, "little")
-      atime_path.write_bytes(timestamp)
+      if self.eviction_enabled:
+        timestamp = time.time_ns().to_bytes(8, "little")
+        atime_path = self.path / f"{key}{_ATIME_SUFFIX}"
+        atime_path.write_bytes(timestamp)
 
     finally:
       if self.eviction_enabled:

tests/lru_cache_test.py

@@ -14,6 +14,7 @@
 
 from __future__ import annotations
 
+import glob
 import importlib.util
 import tempfile
 import time
@@ -153,6 +154,18 @@ def test_max_size(self):
     self.assertIsNone(cache.get("a"))
     self.assertEqual(set(self.path.glob(f"*{_CACHE_SUFFIX}")), set())
 
+  # Check that we don't write access time file when the eviction policy is
+  # disabled. Writing this file can be extremely unperformant and cause
+  # problems on large-scale network storage.
+  def test_no_atime_file(self):
+    cache = LRUCache(self.name, max_size=-1)
+
+    cache.put("a", b"a")
+    self.assertEmpty(glob.glob(self.name + "/*atime*"))
+
+    cache.get("a")
+    self.assertEmpty(glob.glob(self.name + "/*atime*"))
+
 
 if __name__ == "__main__":
   absltest.main(testLoader=jtu.JaxTestLoader())