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Improving design document with references and separating ideas from current implementation.

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Richard Brooksby 2016-03-19 23:31:00 +00:00
parent b8c51be19d
commit b720a44d65
2 changed files with 31 additions and 9 deletions
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2
mps/code/config.h
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@ -675,7 +675,7 @@
* TODO: These settings were determined by trial and error, but should
* be based on measurement of the protection overhead on each
* platform. We know it's extremely different between OS X and
* Windows, for example.
* Windows, for example. See design.mps.write-barrier.improv.by-os.
*
* TODO: Consider basing the count on the amount of time that has
* passed in the mutator rather than the number of scans.

38
mps/design/write-barrier.txt
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@ -39,12 +39,9 @@ logical AND operation.
Write barrier deferral
----------------------
.deferral: The overheads hardware barriers varies widely between
operating systems. On Windows it is very cheap to change memory
protection and to handle protecion faults. On OS X it is very
expensive. The balance between barriers and scanning work is
different. There is no point in spending 1000 CPU units raising a
write barrier to avoid 10 CPU units of scanning cost.
.deferral: Both scanning and the write barrier cost CPU time, and
these must be balanced. There is no point spending 1000 CPU units
raising a write barrier to avoid 10 CPU units of scanning cost.
The MPS balances these costs with write barrier deferral. The write
barrier is not immediately raised when a segment is scanned. Instead,
@ -60,13 +57,38 @@ three events:
3. a barrier hit (``WB_DEFER_HIT``)
See also job003975.
Improvements
------------
.improv.by-os: The overheads hardware barriers varies widely between
operating systems. On Windows it is very cheap to change memory
protection and to handle protecion faults. On OS X it is very
expensive. The balance between barriers and scanning work is
different. We should measure the relative costs and tune the deferral
for each separately.
.improv.balance: Hardware costs of write barriers vary by OS, but
scanning costs vary depending on many factors including client code.
The MPS could dynamically measure these costs, perhaps using fast
cycle counters such as RDTSC, and use this to dynamically balance the
write barrier deferral.
References
----------
.. [job003975] "Poor performance due to imbalance between protection
and scanning costs"; Richard Brooksby; Ravenbrook
Limited; 2016-03-11;
<http://www.ravenbrook.com/project/mps/issue/job003975>.
Document History
----------------
- 2016-03-19 RB_ Created.
- 2016-03-19 RB_ Created during preparation of
branch/2016-03-13/defer-write-barrier for [job003975]_.
.. _RB: http://www.ravenbrook.com/consultants/rb/