JDK-7023931 : PcDescCache::find_pc_desc should not write _last_pc_desc
  • Type: Enhancement
  • Component: hotspot
  • Sub-Component: runtime
  • Affected Version: hs21
  • Priority: P3
  • Status: Closed
  • Resolution: Fixed
  • OS: generic
  • CPU: generic
  • Submitted: 2011-03-02
  • Updated: 2013-11-01
  • Resolved: 2011-04-24
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JDK 7 Other
7Fixed hs21Fixed
Description
A pc descriptor cache is maintained for each nmethod.  The lookup method
find_pc_desc(0 is heavily used during stack walking, and maintains the last
looked-up value in _last_pc_desc.  Unfortunately, _last_pc_desc is a shared
variable, so writes to it can cause the cache line it's in to bounce between
cores and chips, resulting in very high overhead.  On the SPECjvm2008 serial
sub-benchmark, eliminating the write results in a factor of 10 drop in last-
level-cache misses, resulting in a 10% score improvement on some platforms.

This is a useful generic improvement, as stack walking is pervasive during
jvm excecution.
Comments
EVALUATION http://hg.openjdk.java.net/jdk7/hotspot/hotspot/rev/3d5a546351ef
17-03-2011
EVALUATION http://hg.openjdk.java.net/jdk7/hotspot-rt/hotspot/rev/3d5a546351ef
12-03-2011
EVALUATION Ok.
09-03-2011
SUGGESTED FIX diff nmethod.hpp 74a75 > PcDesc* _last_pc_desc; // most recent pc_desc found 77c78 < PcDescCache() { debug_only(_pc_descs[0] = NULL); } --- > PcDescCache() { debug_only(_last_pc_desc = NULL); } 81c82 < PcDesc* last_pc_desc() { return _pc_descs[0]; } --- > PcDesc* last_pc_desc() { return _last_pc_desc; } diff nmethod.cpp 173c173 < int pc_desc_repeats; // number of _pc_descs[0] hits --- > int pc_desc_repeats; // number of _last_pc_desc hits 286c286 < _pc_descs[0] = NULL; // native method; no PcDescs at all --- > _last_pc_desc = NULL; // native method 292,293c292,293 < _pc_descs[0] = initial_pc_desc + 1; // first valid one is after sentinel < for (int i = 1; i < cache_size; i++) --- > _last_pc_desc = initial_pc_desc + 1; // first valid one is after sentinel > for (int i = 0; i < cache_size; i++) 305,307c305,307 < // Step one: Check the most recently added value. < res = _pc_descs[0]; < if (res == NULL) return NULL; // native method; no PcDescs at all --- > // Step one: Check the most recently returned value. > res = _last_pc_desc; > if (res == NULL) return NULL; // native method; no PcDescs at all 314c314 < for (int i = 1; i < cache_size; i++) { --- > for (int i = 0; i < cache_size; i++) { 318a319 > _last_pc_desc = res; // record this cache hit in case of repeat 329c330 < // Update the LRU cache by shifting pc_desc forward. --- > // Update the LRU cache by shifting pc_desc forward: 334a336 > // Note: Do not update _last_pc_desc. It fronts for the LRU cache.
02-03-2011