https://bugs.kde.org/show_bug.cgi?id=446103
Bug ID: 446103
Summary: Memcheck: `--track-origins=yes` causes extreme
slowdowns for large mmap/munmap
Product: valgrind
Version: unspecified
Platform: Other
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: memcheck
Assignee: jsew...@acm.org
Reporter: jsew...@acm.org
Target Milestone: ---
Created attachment 143957
--> https://bugs.kde.org/attachment.cgi?id=143957&action=edit
C test case
The attached test program repeatedly mmaps/munmaps a 2GB chunk of memory.
Without `--track-origins=yes`, it runs in about 0.7 seconds. But with origin
tracking enabled, it takes 95.5 seconds, that is, it runs 137 times slower.
The problem occurs because the 2GB mmaps/munmaps cause Memcheck to perform a
set-address-range-perms operation on the area, changing it to "defined" and
then back to "noaccess". Large-range SARPs are (highly) optimised for the
V/A-bit (normal) Memcheck shadow memory, but they are very inefficient for the
origin-tracking shadow area. Origin-tracking shadows are kept in a 64MB 2-way
set associative cache, which is pretty fast. But lines ejected from that are
kept in a OSet (AVL-tree) based backing store. Because these SARP lengths
greatly exceed the size of the cache, a huge AVL tree is created, which is
very time- and space-inefficient.
This was initially noticed in
https://bugzilla.mozilla.org/show_bug.cgi?id=1742851.
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