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 /*

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  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

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  * published by the Free Software Foundation.

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  * This code is distributed in the hope that it will be useful, but WITHOUT

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  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

  * version 2 for more details (a copy is included in the LICENSE file that

  * accompanied this code).

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  * You should have received a copy of the GNU General Public License version

  * 2 along with this work; if not, write to the Free Software Foundation,

  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

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  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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 class G1CollectedHeap;

 // This file contains the three classes that represent the memory

 // pools of the G1 spaces: G1EdenPool, G1SurvivorPool, and

 // G1OldGenPool. In G1, unlike our other GCs, we do not have a

 // physical space for each of those spaces. Instead, we allocate

 // regions for all three spaces out of a single pool of regions (that

 // pool basically covers the entire heap). As a result, the eden,

 // survivor, and old gen are considered logical spaces in G1, as each

 // is a set of non-contiguous regions. This is also reflected in the

 // way we map them to memory pools here. The easiest way to have done

 // this would have been to map the entire G1 heap to a single memory

 // pool. However, it's helpful to show how large the eden and survivor

 // get, as this does affect the performance and behavior of G1. Which

 // is why we introduce the three memory pools implemented here.

 //

 // The above approach inroduces a couple of challenging issues in the

 // implementation of the three memory pools:

 //

 // 1) The used space calculation for a pool is not necessarily

 // independent of the others. We can easily get from G1 the overall

 // used space in the entire heap, the number of regions in the young

 // generation (includes both eden and survivors), and the number of

 // survivor regions. So, from that we calculate:

 //

 //  survivor_used = survivor_num * region_size

 //  eden_used     = young_region_num * region_size - survivor_used

 //  old_gen_used  = overall_used - eden_used - survivor_used

 //

 // Note that survivor_used and eden_used are upper bounds. To get the

 // actual value we would have to iterate over the regions and add up

 // ->used(). But that'd be expensive. So, we'll accept some lack of

 // accuracy for those two. But, we have to be careful when calculating

 // old_gen_used, in case we subtract from overall_used more then the

 // actual number and our result goes negative.

 //

 // 2) Calculating the used space is straightforward, as described

 // above. However, how do we calculate the committed space, given that

 // we allocate space for the eden, survivor, and old gen out of the

 // same pool of regions? One way to do this is to use the used value

 // as also the committed value for the eden and survivor spaces and

 // then calculate the old gen committed space as follows:

 //

 //  old_gen_committed = overall_committed - eden_committed - survivor_committed

 //

 // Maybe a better way to do that would be to calculate used for eden

 // and survivor as a sum of ->used() over their regions and then

 // calculate committed as region_num * region_size (i.e., what we use

 // to calculate the used space now). This is something to consider

 // in the future.

 //

 // 3) Another decision that is again not straightforward is what is

 // the max size that each memory pool can grow to. One way to do this

 // would be to use the committed size for the max for the eden and

 // survivors and calculate the old gen max as follows (basically, it's

 // a similar pattern to what we use for the committed space, as

 // described above):

 //

 //  old_gen_max = overall_max - eden_max - survivor_max

 //

 // Unfortunately, the above makes the max of each pool fluctuate over

 // time and, even though this is allowed according to the spec, it

 // broke several assumptions in the M&M framework (there were cases

 // where used would reach a value greater than max). So, for max we

 // use -1, which means "undefined" according to the spec.

 //

 // 4) Now, there is a very subtle issue with all the above. The

 // framework will call get_memory_usage() on the three pools

 // asynchronously. As a result, each call might get a different value

 // for, say, survivor_num which will yield inconsistent values for

 // eden_used, survivor_used, and old_gen_used (as survivor_num is used

 // in the calculation of all three). This would normally be

 // ok. However, it's possible that this might cause the sum of

 // eden_used, survivor_used, and old_gen_used to go over the max heap

 // size and this seems to sometimes cause JConsole (and maybe other

 // clients) to get confused. There's not a really an easy / clean

 // solution to this problem, due to the asynchrounous nature of the

 // framework.

 // This class is shared by the three G1 memory pool classes

 // (G1EdenPool, G1SurvivorPool, G1OldGenPool). Given that the way we

 // calculate used / committed bytes for these three pools is related

 // (see comment above), we put the calculations in this class so that

 // we can easily share them among the subclasses.

 class G1MemoryPoolSuper : public CollectedMemoryPool {

 private:

   // It returns x - y if x > y, 0 otherwise.

   // As described in the comment above, some of the inputs to the

   // calculations we have to do are obtained concurrently and hence

   // may be inconsistent with each other. So, this provides a

   // defensive way of performing the subtraction and avoids the value

   // going negative (which would mean a very large result, given that

   // the parameter are size_t).

   static size_t subtract_up_to_zero(size_t x, size_t y) {

     if (x > y) {

       return x - y;

     } else {

       return 0;

     }

   }

 protected:

   G1CollectedHeap* _g1h;

   // Would only be called from subclasses.

   G1MemoryPoolSuper(G1CollectedHeap* g1h,

                     const char* name,

                     size_t init_size,

                     bool support_usage_threshold);

   // The reason why all the code is in static methods is so that it

   // can be safely called from the constructors of the subclasses.

   static size_t undefined_max() {

     return (size_t) -1;

   }

   static size_t overall_committed(G1CollectedHeap* g1h) {

     return g1h->capacity();

   }

   static size_t overall_used(G1CollectedHeap* g1h) {

     return g1h->used_unlocked();

   }

   static size_t eden_space_committed(G1CollectedHeap* g1h);

   static size_t eden_space_used(G1CollectedHeap* g1h);

   static size_t survivor_space_committed(G1CollectedHeap* g1h);

   static size_t survivor_space_used(G1CollectedHeap* g1h);

   static size_t old_space_committed(G1CollectedHeap* g1h);

   static size_t old_space_used(G1CollectedHeap* g1h);

 };

 // Memory pool that represents the G1 eden.

 class G1EdenPool : public G1MemoryPoolSuper {

 public:

   G1EdenPool(G1CollectedHeap* g1h);

   size_t used_in_bytes() {

     return eden_space_used(_g1h);

   }

   size_t max_size() const {

     return undefined_max();

   }

   MemoryUsage get_memory_usage();

 };

 // Memory pool that represents the G1 survivor.

 class G1SurvivorPool : public G1MemoryPoolSuper {

 public:

   G1SurvivorPool(G1CollectedHeap* g1h);

   size_t used_in_bytes() {

     return survivor_space_used(_g1h);

   }

   size_t max_size() const {

     return undefined_max();

   }

   MemoryUsage get_memory_usage();

 };

 // Memory pool that represents the G1 old gen.

 class G1OldGenPool : public G1MemoryPoolSuper {

 public:

   G1OldGenPool(G1CollectedHeap* g1h);

   size_t used_in_bytes() {

     return old_space_used(_g1h);

   }

   size_t max_size() const {

     return undefined_max();

   }

   MemoryUsage get_memory_usage();

 };