Difference between revisions of "Gc info"

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[[Category:Book]]
 
 
Description of the information provided by the <tt>-gc_info</tt> option of the C
 
Description of the information provided by the <tt>-gc_info</tt> option of the C
 
[[Glossary#BackEnd|back-end]] compiler.
 
[[Glossary#BackEnd|back-end]] compiler.
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== Table columns ==
 
== Table columns ==
The main part lists for each reference type and each NATIVE_ARRAY type
+
For each reference type and each NATIVE_ARRAY type,
the following information.
+
the main part lists the following information:
 
=== Number created ===
 
=== Number created ===
 
The number of objects created since program start.
 
The number of objects created since program start.
This is <b>not</b> the number of actually existing objects.
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This is <b>not</b> the number of currently existing objects.
   
 
=== Total size ===
 
=== Total size ===
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The number of free objects left in the "store chunk" of the specified type.
 
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 firstly allocate new objects in the store chunk. Only when there is no store left will the GC starts looking for space elsewhere, which will result in one of the following outcomes:
+
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:
 
<ul>
 
<ul>
 
<li> Previously reclaimed garbage is recycled as a new object.
 
<li> Previously reclaimed garbage is recycled as a new object.
 
<li> A new store chunk is allocated.
 
<li> A new store chunk is allocated.
<li> A garbage collection cycle is started. If garbage of the correct type was reclaimed, garbage is recycled. Else, a new store chunk is allocated.
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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.
 
</ul>
 
</ul>
   
 
== Final line ==
 
== Final line ==
The final line shows synoptic information across all types.
+
The final line shows a summary across all types.
 
=== C-stack=xx ===
 
=== C-stack=xx ===
 
''xx'' is the number of pointers in the stack, i.e. the number of conservative roots.
 
''xx'' is the number of pointers in the stack, i.e. the number of conservative roots.
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''xx'' is the number of memory chunks currently allocated.
 
''xx'' is the number of memory chunks currently allocated.
 
''yyyy'' is the number of chunks that the main table can hold.
 
''yyyy'' is the number of chunks that the main table can hold.
When ''xx'' grows larger than ''yyyy'' the GC needs to reallocate a larger main table
+
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.
(not a very costly operation unless ''yyyy'' is huge).
 
   
 
=== fsoc:xx(free=yy ceil=zz) ===
 
=== fsoc:xx(free=yy ceil=zz) ===
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== Remarks ==
 
== Remarks ==
Currently, there is no information about the memory freed during the last GC cycle,
+
Currently, there is no information about memory freed during the last GC cycle,
i.e. no information about the effect of the GC cycle described in the table.
+
i.e. the table provides no information about the effect of the previous GC cycle.

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.