On 09/19/12 13:56, Stefan Teleman wrote:
This is a proposed fix for the numpunct facet for stdcxx-1056:
FWIW, here is my feed-back. Please correct me if there is something I
did not get right about your proposed patch:
0. Number of reported race conditions is now 0 (zero).
1. No memory leaks in stdcxx (there are memory leaks reported in either
libc or glibc, but there's nothing we can do about these anyway).
2. This fix preserves perfect forwarding in the _numpunct.h header file.
3. This fix eliminates code from facet.cpp and locale_body.cpp which
was creating unnecessary overhead, with the potential of causing
memory corruption, while providing no discernable benefit.
More specifically:
It is not true that there was no eviction policy of cached locales or
facets in stdcxx. Not only cache eviction code existed, and still exists
today, but cache cleanups and resizing were performed periodically,
either when an object's reference count dropped to 0 (zero), or whenever
the number of cached objects fell below sizeof(cache) / 2.
I think you are referring to `live' cache objects and the code which
specifically adjusts the size of the buffer according to the number of
`live' locales and/or facets in it. In that respect I would not call
that eviction because locales and facets with non-zero reference
counters are never evicted.
But anyhoo, this is semantics. Bottom line is the locale/facet buffer
management code follows a principle of economy.
First, the default size of the facets and locales caches was too small:
it was set to 8. I raised this to 32. A direct consequence of this
insufficient default size of 8 was that the cache had to resize itself
very soon after program startup. This cache resize operation consists of:
allocate memory for a new cache, copy the existing cached objects
from the old cache to the new cache, and then delete[] the old cache.
The optimal number is subject to debate. Probably Martin can give an
insight into the reasons for that number. Why did you pick 32 (or is it
64 in your patch) and not any other? Is it something based on your
experience as a user or programmer?
This is a first unnecessary overhead.
A negligible overhead, IMO. The benefits of maintaining a small memory
footprint may be important for some environments. As useful as
principles may be, see above.
Second, and as I mentioned above, whenever the number of cached objects
fell below sizeof(cache) / 2, the cache resized itself, by performing
the same sequence of operations as described above.
This is a second unnecessary overhead.
In this respect you could call every memory allocation and de-allocation
is an overhead. Please keep in mind that this resembles the operations
performed for any sequence containers; how likely is it for a program to
have more locale/facet creation/destruction than strings or vectors
mutations?
Third, cached objects were automatically evicted whenever their reference
count dropped to 0 (zero). There are two consequences to this eviction
policy: if the program needs to re-use an object (facet or locale) which
has been evicted and subsequently destroyed, this object needs then to be
constructed again later on, and subsequently re-inserted into the cache.
This, in turn, would trigger a cache resize, followed by copying and
delete[] of the old cache buffer.
Object eviction followed by destruction followed by reconstruction is
a third unnecessary overhead. Re-inserting a re-constructed object into,
the cache, followed by a potential cache resize involving allocation of
a new buffer, copying pointers from the old cache to the new cache,
followed by delete[] of the old cache is a fourth unnecessary overhead.
What you describe is the classical behavior of reference-counted object
management. Dead, unreferenced objects are destroyed. The overhead of
doing so at each `death' is probably too much; a middle ground approach
is to do it at a factor of 2. The alternative, if a bit extreme, is to
keep all memory ever allocated in a program around just in case it might
be needed again.
Real-life programs tend to reuse locales and/or facets they have created.
There is no point in destroying and evicting these objects simply because
there may be periods when the object isn't referenced at time. The object
is likely to be needed again, later on.
Could you please elaborate a bit on this? Is this your opinion based on
your user and/or programmer experience?
The fix proposed here eliminates the cache eviction and object destruction
policy completely. Once created, objects remain in the cache, even though
they may reside in the cache with no references. This eliminates the
cache resize / copy / delete[] overhead. It also eliminates the overhead
of re-constructing an evicted / destroyed object, if it is needed again
later.
4. Tests and Analysis Results:
4.1. SunPro 12.3 / Solaris / SPARC / Race Conditions Test:
http://s247136804.onlinehome.us/stdcxx-1056-20120919/22.locale.numpunct.mt.sunpro.solaris-sparc.datarace.er.html/index.html
4.2. SunPro 12.3 / Solaris / SPARC / Heap and Memory Leaks Test:
http://s247136804.onlinehome.us/stdcxx-1056-20120919/22.locale.numpunct.mt.sunpro.solaris-sparc.heapcheck.er.html/index.html
4.3. SunPro 12.3 / Linux / Intel / Race Conditions Test:
http://s247136804.onlinehome.us/stdcxx-1056-20120919/22.locale.numpunct.mt.sunpro.linux-intel.datarace.er.html/index.html
4.4. SunPro 12.3 / Linux / Intel / Heap and Memory Leaks Test:
http://s247136804.onlinehome.us/stdcxx-1056-20120919/22.locale.numpunct.mt.sunpro.linux-intel.heapcheck.er.html/index.html
4.5. Intel 2013 / Linux / Intel / Race Conditions Test:
http://s247136804.onlinehome.us/stdcxx-1056-20120919/22.locale.numpunct.mt.intel.linux.datarace.r007ti3.inspxez
4.6. Intel 2013 / Linux / Intel / Heap and Memory Leaks Test:
http://s247136804.onlinehome.us/stdcxx-1056-20120919/22.locale.numpunct.mt.intel.linux.heapcheck.r008mi1.inspxez
5. Source code for this fix:
http://s247136804.onlinehome.us/stdcxx-1056-20120919/_numpunct.h
http://s247136804.onlinehome.us/stdcxx-1056-20120919/facet.cpp
http://s247136804.onlinehome.us/stdcxx-1056-20120919/locale_body.cpp
http://s247136804.onlinehome.us/stdcxx-1056-20120919/punct.cpp
Hey Stefan, are the above also timing the changes?
HTH.
Liviu
