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digitalmars.D.bugs - [Issue 9763] New: contended and contended("groupName")

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=9763

           Summary:  contended and  contended("groupName")
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: bearophile_hugs eml.cc


--- Comment #0 from bearophile_hugs eml.cc 2013-03-19 16:39:52 PDT ---
See for more info:
http://openjdk.java.net/jeps/142
https://blogs.oracle.com/dave/entry/java_contented_annotation_to_help

The idea is to introduce some way in D to specify that some fields in an
struct/object are probably highly contended across processor cores, so the
compiler can pad (or arrange, in objects) to not share cache lines with other
fields, or other structs, that are likely to be independently accessed.

- - - - - - - - - - - - - - - - - - - -

This is from:
http://mail.openjdk.java.net/pipermail/hotspot-dev/2012-November/007309.html


 A. Marking the class as contended:
 
      Contended
     public static class ContendedTest2 {
         private Object plainField1;
         private Object plainField2;
         private Object plainField3;
         private Object plainField4;
     }
 
 ...makes the entire field block to be padded from the both sides:
 (below is the output of new tracing -XX:+PrintFieldLayout)
 
   TestContended$ContendedTest2: field layout
     Entire class is marked contended
       140 --- instance fields start ---
       140 "plainField1" Ljava.lang.Object;
       144 "plainField2" Ljava.lang.Object;
       148 "plainField3" Ljava.lang.Object;
       152 "plainField4" Ljava.lang.Object;
       288 --- instance fields end ---
       288 --- instance ends ---
 
 Note that we use 128 bytes, twice the cache line size on most hardware
 to adjust for adjacent sector prefetchers extending the false sharing
 collisions to two cache lines.
 
 B. Marking the field as contended:
 
     public static class ContendedTest1 {
          Contended
         private Object contendedField1;
         private Object plainField1;
         private Object plainField2;
         private Object plainField3;
         private Object plainField4;
     }
 
 ...pushes the field out of dense block and effectively applies padding:
 
    TestContended$ContendedTest1: field layout
        12 --- instance fields start ---
        12 "plainField1" Ljava.lang.Object;
        16 "plainField2" Ljava.lang.Object;
        20 "plainField3" Ljava.lang.Object;
        24 "plainField4" Ljava.lang.Object;
       156 "contendedField1" Ljava.lang.Object; (contended, group = 0)
       288 --- instance fields end ---
       288 --- instance ends ---
 
 C. Marking multiple fields makes each field padded:
 
     public static class ContendedTest4 {
          Contended
         private Object contendedField1;
 
          Contended
         private Object contendedField2;
 
         private Object plainField3;
         private Object plainField4;
     }
 
 ...pushes both fields with individual padding for each:
 
    TestContended$ContendedTest4: field layout
        12 --- instance fields start ---
        12 "plainField3" Ljava.lang.Object;
        16 "plainField4" Ljava.lang.Object;
       148 "contendedField1" Ljava.lang.Object; (contended, group = 0)
       280 "contendedField2" Ljava.lang.Object; (contended, group = 0)
       416 --- instance fields end ---
       416 --- instance ends ---
 
 *** IV. Contention groups
 
 There are cases where you want to separate the *group* of fields that
 are experiencing contention with everything else but not pairwise. This
 is the usual thing for some of the code updating two fields at once.
 While marking both with  Contended would be sufficient, we can optimize
 the memory footprint by not applying padding between them. In order to
 demarcate these groups, we have the parameter in the annotation
 describing the equivalence class for contention group.
 
 So that:
 
     public static class ContendedTest5 {
          Contended("updater1")
         private Object contendedField1;
 
          Contended("updater1")
         private Object contendedField2;
 
          Contended("updater2")
         private Object contendedField3;
 
         private Object plainField5;
         private Object plainField6;
     }
 
 ...is laid out as:
 
    TestContended$ContendedTest5: field layout
        12 --- instance fields start ---
        12 "plainField5" Ljava.lang.Object;
        16 "plainField6" Ljava.lang.Object;
       148 "contendedField1" Ljava.lang.Object; (contended, group = 12)
       152 "contendedField2" Ljava.lang.Object; (contended, group = 12)
       284 "contendedField3" Ljava.lang.Object; (contended, group = 15)
       416 --- instance fields end ---
       416 --- instance ends ---
 
 Note $contendedField1 and $contendedField2 are padded from everything
 else, but still densely packed with each other.

- - - - - - - - - - - - - - - - - - - - A design for D is similar. For structs the problem is solved with padding between fields or between structs, while in class instances it's solved with padding and/or field reordering. The syntax is similar: struct Test1 { contended int field1; int field2; contended int field3; } contended final class Test2 { contended int field1; int field2; contended int field3; } contended struct Test3 { int field1; int field2; } contended final class Test4 { private Object field1; private Object field2; } - - - - - - - - - - - - - - - - - - - - Contention groups in D: struct Test5 { contended("group1") int field1; contended("group1") int field2; contended("group2") int field3; private int field4; private int field5; } - - - - - - - - - - - - - - - - - - - - I think the D compiler should ignore contended for const/immutable fields: alias T = const(int); struct Test6 { contended T field1; int field2; } static assert(Test6.sizeof == 8); - - - - - - - - - - - - - - - - - - - - An alternative syntax is to use align: align(cache_line) align(cache_line, "group1") Like this: struct Test7 { align(cache_line) int field1; int field2; align(cache_line) int field3; } align(cache_line) class Test8 { private Object field1; private Object field2; } struct Test9 { align(cache_line, "group1") int field1; align(cache_line, "group1") int field2; align(cache_line, "group2") int field3; private int field4; private int field5; } But contended is more explicit in its purpose. - - - - - - - - - - - - - - - - - - - - Cache lines contention is a well known phenomenon. This is a small example. The code is adapted from: http://rosettacode.org/wiki/Atomic_updates#D // start atomic_updates.d code. import std.stdio: writeln; import std.conv: text; import std.random: uniform, Xorshift; import std.algorithm: min, max; import std.parallelism: task; import core.thread: Thread; import core.sync.mutex: Mutex; import core.time: dur; __gshared uint transfersCount; final class Buckets(size_t nBuckets) if (nBuckets > 0) { alias TBucketValue = uint; private static struct Bucket { TBucketValue value; Mutex mtx; //uint[14] _voidPadding = void; alias value this; } private Bucket[nBuckets] buckets; private bool running; public this() { this.running = true; foreach (ref b; buckets) b = Bucket(uniform(0, 100), new Mutex()); } public TBucketValue opIndex(in size_t index) const pure nothrow { return buckets[index]; } public void transfer(in size_t from, in size_t to, in TBucketValue amount) { immutable low = min(from, to); immutable high = max(from, to); buckets[low].mtx.lock(); buckets[high].mtx.lock(); scope(exit) { buckets[low].mtx.unlock(); buckets[high].mtx.unlock(); } immutable realAmount = min(buckets[from].value, amount); buckets[from] -= realAmount; buckets[to ] += realAmount; transfersCount++; } property size_t length() const pure nothrow { return this.buckets.length; } void toString(in void delegate(const(char)[]) sink) { TBucketValue total = 0; foreach (ref b; buckets) { b.mtx.lock(); total += b; } scope(exit) foreach (ref b; buckets) b.mtx.unlock(); sink(text(buckets)); sink(" "); sink(text(total)); } } void randomize(size_t N)(Buckets!N data) { immutable maxi = data.length - 1; auto rng = Xorshift(1); while (data.running) { immutable i = uniform(0, maxi, rng); immutable j = uniform(0, maxi, rng); immutable amount = uniform(0, 20, rng); data.transfer(i, j, amount); } } void equalize(size_t N)(Buckets!N data) { immutable maxi = data.length - 1; auto rng = Xorshift(1); while (data.running) { immutable i = uniform(0, maxi, rng); immutable j = uniform(0, maxi, rng); immutable a = data[i]; immutable b = data[j]; if (a > b) data.transfer(i, j, (a - b) / 2); else data.transfer(j, i, (b - a) / 2); } } void display(size_t N)(Buckets!N data) { foreach (immutable _; 0 .. 10) { writeln(transfersCount, " ", data); transfersCount = 0; Thread.sleep(dur!"msecs"(1000)); } data.running = false; } void main() { writeln("N. transfers, buckets, buckets sum:"); auto data = new Buckets!20(); task!randomize(data).executeInNewThread(); task!equalize(data).executeInNewThread(); task!display(data).executeInNewThread(); } // end atomic_updates.d code. Timings by nazriel on IRC, done with dmd on an Intel i5 2,4ghz, arch linux x86_64 with: -release -inline -O -noboundscheck Without padding in struct Bucket: N. transfers, buckets, buckets sum: 85 [0, 5, 0, 51, 5, 25, 89, 0, 147, 93, 47, 2, 76, 43, 76, 69, 0, 62, 67, 51] 908 6246828 [35, 51, 38, 25, 48, 88, 11, 23, 39, 54, 92, 13, 69, 48, 29, 48, 52, 26, 68, 51] 908 6322759 [60, 40, 43, 55, 44, 39, 49, 61, 38, 44, 51, 42, 18, 37, 34, 44, 50, 48, 60, 51] 908 6103135 [29, 63, 74, 44, 44, 48, 44, 53, 35, 62, 35, 44, 44, 44, 34, 47, 31, 40, 42, 51] 908 6567254 [36, 39, 42, 49, 70, 36, 10, 28, 51, 39, 55, 42, 35, 72, 42, 65, 35, 42, 69, 51] 908 6274262 [55, 63, 27, 40, 31, 40, 41, 32, 20, 44, 45, 95, 42, 54, 45, 38, 76, 40, 29, 51] 908 6171490 [39, 24, 61, 56, 42, 51, 39, 56, 62, 43, 72, 39, 56, 13, 32, 67, 45, 48, 12, 51] 908 6211137 [29, 39, 36, 70, 16, 41, 48, 35, 74, 20, 65, 59, 48, 48, 43, 59, 54, 30, 43, 51] 908 6202907 [51, 16, 80, 36, 25, 42, 43, 58, 48, 41, 41, 35, 80, 49, 54, 24, 40, 44, 50, 51] 908 6350466 [50, 35, 37, 66, 46, 65, 38, 48, 52, 47, 51, 46, 20, 37, 34, 46, 51, 38, 50, 51] 908 With padding in struct Bucket: N. transfers, buckets, buckets sum: 112 [22, 41, 41, 54, 54, 58, 52, 32, 56, 53, 49, 64, 59, 65, 56, 74, 54, 59, 66, 55] 1064 6698768 [56, 58, 54, 29, 38, 54, 58, 55, 54, 72, 26, 57, 83, 51, 49, 45, 56, 57, 57, 55] 1064 8371235 [53, 53, 53, 53, 52, 53, 53, 54, 53, 54, 53, 52, 53, 54, 53, 54, 53, 53, 53, 55] 1064 7680510 [73, 71, 16, 53, 20, 55, 84, 57, 55, 53, 49, 55, 57, 50, 52, 68, 53, 35, 53, 55] 1064 6675895 [45, 32, 79, 46, 48, 29, 65, 79, 13, 79, 0, 79, 61, 65, 40, 63, 65, 61, 60, 55] 1064 7865702 [31, 63, 58, 76, 54, 43, 49, 51, 51, 63, 54, 29, 47, 66, 54, 62, 54, 56, 48, 55] 1064 8068515 [51, 69, 49, 54, 54, 59, 51, 58, 44, 46, 58, 56, 48, 57, 59, 48, 38, 56, 54, 55] 1064 6718293 [40, 55, 52, 55, 77, 51, 40, 57, 87, 37, 56, 56, 49, 69, 65, 17, 13, 54, 79, 55] 1064 6512280 [53, 34, 21, 69, 47, 69, 43, 25, 61, 43, 63, 52, 97, 52, 50, 52, 49, 78, 51, 55] 1064 7408234 [51, 44, 54, 56, 31, 55, 34, 59, 72, 69, 56, 50, 58, 70, 45, 67, 48, 41, 49, 55] 1064 The numbers in the first column like 6274262 represent the number of updated in a second. With the padding there is less cache lines contention between two cores, so it performs more atomic updated in a second, like 7408234. That manual padding in Bucket is similar to: contended private static struct Bucket { TBucketValue value; Mutex mtx; alias value this; } One difference is that in the Java example the padding is on both sides, and it's twice larger, as explained:
 Note that we use 128 bytes, twice the cache line size on most hardware
 to adjust for adjacent sector prefetchers extending the false sharing
 collisions to two cache lines.

- - - - - - - - - - - - - - - - - - - - -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
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parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=9763



--- Comment #1 from bearophile_hugs eml.cc 2013-03-20 11:53:59 PDT ---
A current workaround is to use align (with a value, because of Issue 9766 ):


align(128) struct Test3 {
    int field1;
    int field2;
}
pragma(msg, "Test3:");
pragma(msg, Test3.field1.offsetof);
pragma(msg, Test3.field2.offsetof);
pragma(msg, "Total size:");
pragma(msg, Test3.sizeof);
pragma(msg, "");


The print shows there is trailing padding (no leading padding):

Test3:
0u
4u
Total size:
128u


Adding align(128) on some fields of struct/object allows to introduce
intermediate padding, but it's tricky to get all the padding right. But
 contended adapts automatically the padding needed on different CPUs and makes
the creation of spaces and groups simpler.

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