Re: [mat-dev] Viewing Discarded Objects

[Kevin Grigorenko <kevin.grigorenko@xxxxxxxxxx>]

Hey Andrew, My only concern is whether this will drive a lot more object 
creation and garbage collection with the ObjectReference instances and 
whether that will impact performance much?

--
Kevin Grigorenko
IBM App Platform SWAT




From:   Andrew Johnson <andrew_johnson@xxxxxxxxxx>
To:     "Memory list" <mat-dev@xxxxxxxxxxx>
Date:   05/21/2021 12:02 PM
Subject:        [EXTERNAL] [mat-dev] Viewing Discarded Objects
Sent by:        "mat-dev" <mat-dev-bounces@xxxxxxxxxxx>



Memory Analyzer has the facility to copy with huge heaps (>2^31 objects) 
or more than the heap can manage by discarding objects. This can be 
useful, does reduce the number of objects, but the object graph is 
modified, and even if the leak ZjQcmQRYFpfptBannerStart 
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Memory Analyzer has the facility to copy with huge heaps (>2^31 objects) 
or more than the heap can manage by discarding objects. 

This can be useful, does reduce the number of objects, but the object 
graph is modified, and even if the leak can be spotted it is harder to 
understand because some of the strings giving names of objects might have 
been discarded. 

I have a work-around for the latter problem. Generally MAT using an int 
index to represent an object. However, once you have an IObject you can 
find its address. You can read fields (and array entries using "[13]" 
etc.) and call getOutboundReferences(). These return a ObjectReference 
object, which could represent an unindexed (discarded) object as it holds 
the object ID, the address and the snapshot. 

I didn't want to change the API as this is still experimental. There isn't 
a way of getting unindexed objects by address from directly from a 
snapshot, or from a parser via IHeapObjectReader 

Currently 
int objectId = snapshot.mapAddressToId(address); 
IObject object = snapshot.getObject(objectId); 

instead I suggest the following: 

ObjectReference ref = new ObjectReference(snapshot, address); 
IObject object = ref.getObject() 

I also wanted the inspector view to show unindexed objects and fields of 
those objects and to be able to inspect those fields and go into them. 
I also wanted some support for OQL. 

To get this to work I need a way for the parser to construct this new 
object.
https://help.eclipse.org/2021-03/topic/org.eclipse.mat.ui.help/doc/org/eclipse/mat/parser/IObjectReader.html 

void 
close() 
tidy up when snapshot no longer required 
<A> A 
getAddon(Class<A> addon) 
Get additional information about the snapshot 
void 
open(ISnapshot snapshot) 
Open the dump file associated with the snapshot 
IObject 
read(int objectId, ISnapshot snapshot) 
Get detailed information about an object 
long[] 
readObjectArrayContent(ObjectArrayImpl array, int offset, int length) 
Get detailed information about a object array 
Object 
readPrimitiveArrayContent(PrimitiveArrayImpl array, int offset, int 
length) 
Get detailed information about a primitive array
To construct this object my idea is that the parser needs to provide a 
getAddon for ObjectReference, returning a parser specific version. The 
parser can then override getObject and retrieve an object at the address 
which has been inserted into the object. So the base ObjectReference tries 
to get the object ID and call snapshot.getObject(). If there is no object 
ID, then it constructs a proxy ObjectReference from the parser, inserts 
the required object address, and lets the parser return the object.
I have made the changes - but there is still some time to improve or 
revert them so give it a try with snapshots with discarded objects.
--
Andrew Johnson


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