Opened 7 years ago

Closed 5 years ago

#15825 closed Bug (fixed)

File-based caching doesn't cull truly randomly

Reported by: gkuenning Owned by: nobody
Component: Core (Cache system) Version: master
Severity: Normal Keywords:
Cc: Jaap Roes Triage Stage: Accepted
Has patch: yes Needs documentation: no
Needs tests: no Patch needs improvement: no
Easy pickings: no UI/UX: no


The culling algorithm in uses an essentially random culling method, which is well known to lead to an approximation to LRU. However, the randomness is improperly applied. For example, consider a frequently-accessed page that, by luck, happens to sort first in the file list (due to an alphatbetically low hash value). The culling code will always delete this file, while a truly random algorithm would only do so 1/n of the time.

The fix is easy: at approximately line 131 in, "== 0" should be replaced with "== ranno" where ranno is chosen in the immediately preceding line with:

ranno = random.randint(0, self._cull_frequency)

Attachments (2)

patch_for_15825.diff (2.7 KB) - added by Nikolay Zakharov 7 years ago.
Initial patch against trunk (r16347)
merged_patch.diff (4.1 KB) - added by Nikolay Zakharov 7 years ago.
Patch for both #15825 and #15806 (as they address same functions)

Download all attachments as: .zip

Change History (11)

comment:1 Changed 7 years ago by Carl Meyer

Easy pickings: unset
Triage Stage: UnreviewedAccepted
Version: 1.2SVN

The referenced line of code in trunk is 123, or doomed = [os.path.join(self._dir, k) for (i, k) in enumerate(filelist) if i % self._cull_frequency == 0]

comment:2 Changed 7 years ago by Luke Plant

If this is fixed, please note that the tests will need some subtle fixing, particularly FileBasedCacheTests.test_cull. The tests check for a certain number deleted, but because of the way that files are grouped into folders in the file based backend, if this is randomized the number deleted can change. The 'sorted' call just a few lines above this line was added because of this issue, to fix a difference that appeared on different platforms, #14250.

comment:3 Changed 7 years ago by Jacob

See also #15806. Not exactly related, but right around in that same part of code and probably worth fixing at the same time.

Changed 7 years ago by Nikolay Zakharov

Attachment: patch_for_15825.diff added

Initial patch against trunk (r16347)

Changed 7 years ago by Nikolay Zakharov

Attachment: merged_patch.diff added

Patch for both #15825 and #15806 (as they address same functions)

comment:4 Changed 7 years ago by Nikolay Zakharov

Has patch: set
UI/UX: unset

I've added patch against trunk AND patch that also fixes #15806 (it's actually just a result of merge) to avoid merging conflicts.

I've removed sorted() cause it's make no sense with random culling. I've also replaced nested for that traverse directories to delete paths by shutils.rmtree, as it does the same.

Tests changed in two ways: "cull%d" became "cull-%d" and initial inserts count for cull test was reduced from 49 to 34, to have 34 unique 2-char prefixes after hashing. It's actually a bit rude but in fact, #14250 was about as rude as this.

I see two other ways to solve last problem with tests:

  • change length of directory name from 2 to 3, so there will be 4096 possible prefixes (conceptually, same hack but from different pov);
  • change the way of culling: delete exactly _num_entries_cull_frequency FILES, not directories (bad thing, cause it will not scale with big cache sizes).

Waiting for review.

comment:5 Changed 5 years ago by anonymous

This seems to have been forgotten. Has the patch rotted?

comment:6 Changed 5 years ago by Susan Tan

I ran the applied patch against the full test suite, which produced a failure. Stack trace here:

FAIL: test_cull (cache.tests.FileBasedCacheTests)

Traceback (most recent call last):

File "../django/tests/cache/", line 1085, in test_cull

self.perform_cull_test(35, 24)

File "../django/tests/cache/", line 495, in perform_cull_test

self.assertEqual(count, final_count)

AssertionError: 23 != 24

Last edited 5 years ago by Susan Tan (previous) (diff)

comment:7 Changed 5 years ago by Jaap Roes

Cc: Jaap Roes added

comment:8 Changed 5 years ago by Jaap Roes

Created a pull request that addresses this issue: (also #20536)

comment:9 Changed 5 years ago by Anssi Kääriäinen <akaariai@…>

Resolution: fixed
Status: newclosed

In 7be638390e18fcbfaaed638f9908673360c280d3:

Fixed #20536 -- rewrite of the file based cache backend

  • Safer for use in multiprocess environments
  • Better random culling
  • Cache files use less disk space
  • Safer delete behavior

Also fixed #15806, fixed #15825.

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