Difference between revisions of "Gc info"
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| + | Description of the information provided by the <tt>-gc_info</tt> option of the C |
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| − | == Column "Number created" == |
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| + | [[Glossary#BackEnd|back-end]] compiler. |
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| − | This is the number of objects (of the specified type) created since program start, (i.e. this figure is accumulative). This is <b>not</b> the number of actually existing objects. |
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| + | The information is printed to ''std_output'' after each GC cycle. |
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| − | == |
+ | == Table columns == |
| + | For each reference type and each NATIVE_ARRAY type, |
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| − | This is the same as "Number created", except the unit is bytes. |
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| + | the main part lists the following information: |
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| + | === Number created === |
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| + | The number of objects created since program start. |
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| + | This is <b>not</b> the number of currently existing objects. |
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| + | === Total size === |
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| − | == Column "Store left": I conjecture this means the memory |
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| + | The same as '''Number created''' but the unit is bytes. |
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| − | > allocated but not occupied after the GC cycle, and the unit |
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| − | > is that of column "Total size". Is this right? |
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| + | === Store left === |
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| − | == Where is the information about the freed memory? == |
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| + | The number of free objects left in the "store chunk" of the specified type. |
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| − | i.e. about the effect of the last GC cycle? |
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| + | This number is mainly interesting because it is zero for the type that triggered the collection cycle. Here's why: the GC will initially allocate new objects in the store chunk. Only when there is no store left will the GC start looking for space elsewhere, which will result in one of the following outcomes: |
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| − | == What does the cryptic final lines mean? == |
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| + | <ul> |
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| − | * C-stack=xx |
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| + | <li> Previously reclaimed garbage is recycled as a new object. |
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| − | * main-table=xx/yyyy |
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| + | <li> A new store chunk is allocated. |
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| − | * fsoc:xx(free=yy ceil=zz) |
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| + | <li> A garbage collection cycle is started. If garbage of the correct type was reclaimed, garbage is recycled, otherwise a new store chunk is allocated. |
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| − | * rsoc:xx(ceil=yy) |
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| + | </ul> |
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| + | |||
| + | == Final line == |
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| + | The final line shows a summary across all types. |
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| + | === C-stack=xx === |
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| + | ''xx'' is the number of pointers in the stack, i.e. the number of conservative roots. |
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| + | |||
| + | === main-table=xx/yyyy === |
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| + | ''xx'' is the number of memory chunks currently allocated. |
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| + | ''yyyy'' is the number of chunks that the main table can hold. |
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| + | When ''xx'' grows larger than ''yyyy'' the garbage collector needs to reallocate a larger main table, which is not a very costly operation unless ''yyyy'' is huge. |
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| + | |||
| + | === fsoc:xx(free=yy ceil=zz) === |
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| + | ''xx'' is the number of memory chunks currently allocated to fixed-size objects |
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| + | (all reference objects except NATIVE_ARRAYs). |
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| + | ''yy'' is the number of free memory chunks, that is chunks that are devoted to fixed-size objects, but that do not hold any objects. |
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| + | ''zz'' is the number of chunks that can be allocated to fixed-size objects |
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| + | before a GC cycle is triggered. |
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| + | |||
| + | When the GC needs to allocate a new object and it has exhausted that type's store chunk |
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| + | and reclaimed garbage, it tries to find a new store chunk for that type. |
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| + | If ''yy'' is non-null, a free memory chunk is converted to a store chunk. |
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| + | Else, if ''xx'' is smaller than ''zz'' then a chunk is allocated from the system. |
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| + | Else, a GC cycle is started. |
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| + | |||
| + | === rsoc:xx(ceil=yy) === |
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| + | ''xx'' is the number of memory chunks currently allocated to resizable objects (NATIVE_ARRAYs). |
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| + | ''yy'' is the number of chunks that can be allocated to resizable objects |
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| + | before a GC cycle is triggered. |
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| + | |||
| + | == Remarks == |
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| + | Currently, there is no information about memory freed during the last GC cycle, |
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| + | i.e. the table provides no information about the effect of the previous GC cycle. |
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Latest revision as of 08:58, 17 June 2016
Description of the information provided by the -gc_info option of the C back-end compiler. The information is printed to std_output after each GC cycle.
Table columns
For each reference type and each NATIVE_ARRAY type, the main part lists the following information:
Number created
The number of objects created since program start. This is not the number of currently existing objects.
Total size
The same as Number created but the unit is bytes.
Store left
The number of free objects left in the "store chunk" of the specified type.
This number is mainly interesting because it is zero for the type that triggered the collection cycle. Here's why: the GC will initially allocate new objects in the store chunk. Only when there is no store left will the GC start looking for space elsewhere, which will result in one of the following outcomes:
- Previously reclaimed garbage is recycled as a new object.
- A new store chunk is allocated.
- A garbage collection cycle is started. If garbage of the correct type was reclaimed, garbage is recycled, otherwise a new store chunk is allocated.
Final line
The final line shows a summary across all types.
C-stack=xx
xx is the number of pointers in the stack, i.e. the number of conservative roots.
main-table=xx/yyyy
xx is the number of memory chunks currently allocated. yyyy is the number of chunks that the main table can hold. When xx grows larger than yyyy the garbage collector needs to reallocate a larger main table, which is not a very costly operation unless yyyy is huge.
fsoc:xx(free=yy ceil=zz)
xx is the number of memory chunks currently allocated to fixed-size objects (all reference objects except NATIVE_ARRAYs). yy is the number of free memory chunks, that is chunks that are devoted to fixed-size objects, but that do not hold any objects. zz is the number of chunks that can be allocated to fixed-size objects before a GC cycle is triggered.
When the GC needs to allocate a new object and it has exhausted that type's store chunk and reclaimed garbage, it tries to find a new store chunk for that type. If yy is non-null, a free memory chunk is converted to a store chunk. Else, if xx is smaller than zz then a chunk is allocated from the system. Else, a GC cycle is started.
rsoc:xx(ceil=yy)
xx is the number of memory chunks currently allocated to resizable objects (NATIVE_ARRAYs). yy is the number of chunks that can be allocated to resizable objects before a GC cycle is triggered.
Remarks
Currently, there is no information about memory freed during the last GC cycle, i.e. the table provides no information about the effect of the previous GC cycle.