1.1 --- a/src/share/vm/runtime/synchronizer.cpp Wed Aug 13 10:44:50 2014 +0200
1.2 +++ b/src/share/vm/runtime/synchronizer.cpp Fri Mar 17 03:39:23 2017 -0700
1.3 @@ -437,19 +437,22 @@
1.4 // Hash Code handling
1.5 //
1.6 // Performance concern:
1.7 -// OrderAccess::storestore() calls release() which STs 0 into the global volatile
1.8 -// OrderAccess::Dummy variable. This store is unnecessary for correctness.
1.9 -// Many threads STing into a common location causes considerable cache migration
1.10 -// or "sloshing" on large SMP system. As such, I avoid using OrderAccess::storestore()
1.11 -// until it's repaired. In some cases OrderAccess::fence() -- which incurs local
1.12 -// latency on the executing processor -- is a better choice as it scales on SMP
1.13 -// systems. See http://blogs.sun.com/dave/entry/biased_locking_in_hotspot for a
1.14 -// discussion of coherency costs. Note that all our current reference platforms
1.15 -// provide strong ST-ST order, so the issue is moot on IA32, x64, and SPARC.
1.16 +// OrderAccess::storestore() calls release() which at one time stored 0
1.17 +// into the global volatile OrderAccess::dummy variable. This store was
1.18 +// unnecessary for correctness. Many threads storing into a common location
1.19 +// causes considerable cache migration or "sloshing" on large SMP systems.
1.20 +// As such, I avoided using OrderAccess::storestore(). In some cases
1.21 +// OrderAccess::fence() -- which incurs local latency on the executing
1.22 +// processor -- is a better choice as it scales on SMP systems.
1.23 +//
1.24 +// See http://blogs.oracle.com/dave/entry/biased_locking_in_hotspot for
1.25 +// a discussion of coherency costs. Note that all our current reference
1.26 +// platforms provide strong ST-ST order, so the issue is moot on IA32,
1.27 +// x64, and SPARC.
1.28 //
1.29 // As a general policy we use "volatile" to control compiler-based reordering
1.30 -// and explicit fences (barriers) to control for architectural reordering performed
1.31 -// by the CPU(s) or platform.
1.32 +// and explicit fences (barriers) to control for architectural reordering
1.33 +// performed by the CPU(s) or platform.
1.34
1.35 struct SharedGlobals {
1.36 // These are highly shared mostly-read variables.
1.37 @@ -1636,7 +1639,55 @@
1.38 }
1.39
1.40 //------------------------------------------------------------------------------
1.41 -// Non-product code
1.42 +// Debugging code
1.43 +
1.44 +void ObjectSynchronizer::sanity_checks(const bool verbose,
1.45 + const uint cache_line_size,
1.46 + int *error_cnt_ptr,
1.47 + int *warning_cnt_ptr) {
1.48 + u_char *addr_begin = (u_char*)&GVars;
1.49 + u_char *addr_stwRandom = (u_char*)&GVars.stwRandom;
1.50 + u_char *addr_hcSequence = (u_char*)&GVars.hcSequence;
1.51 +
1.52 + if (verbose) {
1.53 + tty->print_cr("INFO: sizeof(SharedGlobals)=" SIZE_FORMAT,
1.54 + sizeof(SharedGlobals));
1.55 + }
1.56 +
1.57 + uint offset_stwRandom = (uint)(addr_stwRandom - addr_begin);
1.58 + if (verbose) tty->print_cr("INFO: offset(stwRandom)=%u", offset_stwRandom);
1.59 +
1.60 + uint offset_hcSequence = (uint)(addr_hcSequence - addr_begin);
1.61 + if (verbose) {
1.62 + tty->print_cr("INFO: offset(_hcSequence)=%u", offset_hcSequence);
1.63 + }
1.64 +
1.65 + if (cache_line_size != 0) {
1.66 + // We were able to determine the L1 data cache line size so
1.67 + // do some cache line specific sanity checks
1.68 +
1.69 + if (offset_stwRandom < cache_line_size) {
1.70 + tty->print_cr("WARNING: the SharedGlobals.stwRandom field is closer "
1.71 + "to the struct beginning than a cache line which permits "
1.72 + "false sharing.");
1.73 + (*warning_cnt_ptr)++;
1.74 + }
1.75 +
1.76 + if ((offset_hcSequence - offset_stwRandom) < cache_line_size) {
1.77 + tty->print_cr("WARNING: the SharedGlobals.stwRandom and "
1.78 + "SharedGlobals.hcSequence fields are closer than a cache "
1.79 + "line which permits false sharing.");
1.80 + (*warning_cnt_ptr)++;
1.81 + }
1.82 +
1.83 + if ((sizeof(SharedGlobals) - offset_hcSequence) < cache_line_size) {
1.84 + tty->print_cr("WARNING: the SharedGlobals.hcSequence field is closer "
1.85 + "to the struct end than a cache line which permits false "
1.86 + "sharing.");
1.87 + (*warning_cnt_ptr)++;
1.88 + }
1.89 + }
1.90 +}
1.91
1.92 #ifndef PRODUCT
1.93
