RFR (XL) 8031320: Use Intel RTM instructions for locks
vladimir.kozlov at oracle.com
Mon Mar 17 19:11:43 UTC 2014
The Intel architectures codenamed Haswell has support for RTM
(Restricted Transactional Memory) instructions xbegin, xabort, xend and
xtest as part of Intel Transactional Synchronization Extension (TSX).
The xbegin and xend instructions enclose a set of instructions to be
executed as a transaction. If no conflict found during execution of the
transaction, the memory and register modifications are committed
together at xend. xabort instruction can be used for explicit abort of
transaction and xtest to check if we are in transaction.
RTM is useful for highly contended locks with low conflict in the
critical region. The highly contended locks don't scale well otherwise
but with RTM they show good scaling. RTM allows using coarse grain
locking for applications. Also for lightly contended locks which are
used by different threads RTM can reduce cache line ping pong and
thereby show performance improvement too.
Generate RTM locking code for all inflated locks when "UseRTMLocking"
option is on with normal locking as fall back mechanism. On abort or
lock busy the lock will be retried a fixed number of times as specified
by "RTMRetryCount" option. The locks which abort too often can be auto
tuned or manually tuned.
Auto-tuning can be done using "UseRTMDeopt" flag which will add an abort
ratio calculation code for each lock. The abort ratio will be calculated
after "RTMAbortThreshold" aborts are encountered.
With "UseRTMDeopt" if the aborts ratio reaches "RTMAbortRatio" the
nmethod containing the lock will be deoptimized and recompiled with all
locks as normal (stack) locks. If the abort ratio continues to remain
low after "RTMLockingThreshold" attempted locks, then the method will be
deoptimized and recompiled with all locks as RTM locks without abort
ratio calculation code. The abort ratio calculation can be delayed by
specifying -XX:RTMLockingCalculationDelay=<millisec> flag.
Deoptimization of nmethod is done by adding an uncommon trap at the
beginning of the code which checks rtm state field in MDO which is
modified by the abort calculation code.
For manual tuning the abort statistics for each lock could be provided
to a user using "PrintPreciseRTMLockingStatistics" diagnostic flag.
Based on the abort statistics users can create a .hotspot_compiler file
or use -XX:CompileCommand=<option> flag to specify for which methods
disable RTM locking using <option> "NoRTMLockEliding" or always enable
RTM locking using <option> "UseRTMLockEliding".
The abort calculation and statistic collection are done using
RTMLockingCounters wrapped into RTMLockingNamedCounter counters which
are generated for each lock. To reduce burden on cache line RTM lock
total counter is updated randomly with RTMTotalCountIncrRate rate.
Note, both auto and manually tuning is done for whole method. There is
no a mechanism to tune an individual lock.
RTM locking can be used for normal (stack) locks by specifying
RTM locking code requires that biased locking is switched off because it
conflicts with it. RTM locking is most useful when there is high lock
contention and low data contention. With high lock contention the lock
is usually inflated and biased locking is not suitable for that case
It was requested that this code did not affect other platforms. For that
the most of the code is put under #if INCLUDE_RTM_OPT which is defined
only for X86 and C2 and not EMBEDDED.
All new RTM flags are declared as experimental and require to specify
SQE did full testing on these changes. Additional tests were developed.
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