Hi Eugeniu,
I should have been more clear around that your understanding of the
numbers are correct. But as Thomas also responded, these are estimates
and we might have to start a GC due to other circumstances.
See more below.
On 2020-03-09 11:32, Eugeniu Rabii wrote:
Hello Stefan,
I know these are humongous allocations, the 2 MB was chosen on purpose
(I could have chose 1 MB too, I know).
The first GC (0 - young collection) is actually the result of the
allocation of those humongous Objects.
Because the humongous allocation happened, a concurrent GC was triggered
(GC (1)) that triggers the young collection first (GC (0)); these are
concepts I do seem do get.
My question here is different. After the young collection is done, there
are entries like this in logs:
[0.071s][info ][gc,heap ] GC(0) Eden regions: 1->0(9)
[0.071s][info ][gc,heap ] GC(0) Survivor regions: 0->1(2)
The way I read it it is: there were 1 Eden Regions before the
collection; everything was cleared from them (that zero) and the
heuristics just said that the next cycle should have 9 Eden Regions.
Correct, but this is an estimate and we might have to GC before we fill
up the 9 young regions. For example if there are a lot of humongous
allocations. The humongous allocations are as I mentioned treated
differently and aren't considered young.
Same explanation would happen for Survivor Regions. As such there would
be : 11 young, 2 survivor.
I am expecting the third cycle (GC (2)) to start with :
[0.076s][debug][gc,heap ] GC(2) Heap before GC invocations=2
(full 0): .....
[0.076s][debug][gc,heap ] GC(2) region size 1024K, 11 young
(2048K), 2 survivors (1024K)
Instead it prints:
[0.076s][debug][gc,heap ] GC(2) region size 1024K, 2 young
(2048K), 1 survivors (1024K)
Does this makes it a better explanation now?
Your expectation is correct, and if GC(2) isn't caused by a humongous
allocation this is unexpected behavior. It would help a lot if you could
post more of your log, especially the cause for GC(2).
Thanks,
Stefan
Thank you,
Eugene.
On 3/9/20 4:14 AM, Stefan Johansson wrote:
Hi Eugeniu,
The second GC is most likely also caused by having many humongous
allocations. This was the cause GC(0) as well, and since your
application only allocates large (humongous) objects it will not use a
lot of space for other objects.
If you are not familiar with the concept of humongous objects in G1,
these are objects that are to large to be allocated in the normal fast
path. They are instead allocated in separate regions. This requires
some special handling and that's the reason we trigger GCs more
quickly if a lot of such objects are allocated. In your setup the
region size will be 1MB so all objects larger than 500KB will be
considered humongous.
Hope this helps,
StefanJ
On 2020-03-08 06:01, Eugeniu Rabii wrote:
Hello,
I have a very simple program that constantly allocates some byte
arrays (of 2 MB) each (running with the latest jdk-13). I run it with :
-Xms20M
-Xmx20M
-Xmn10M
"-Xlog:heap*=debug" "-Xlog:gc*=debug" "-Xlog:ergo*=debug"
For example:
public static void main(String[] args) {
while (true) {
System.out.println(invokeMe());
}
}
public static int invokeMe() {
int x = 1024;
int factor = 2;
byte[] allocation1 = new byte[factor * x * x];
allocation1[2] = 3;
byte[] allocation2 = new byte[factor * x * x];
byte[] allocation3 = new byte[factor * x * x];
byte[] allocation4 = new byte[factor * x * x];
return Arrays.hashCode(allocation1) ^
Arrays.hashCode(allocation2)
^ Arrays.hashCode(allocation3) ^
Arrays.hashCode(allocation4);
}
In logs, I see something that is puzzling me:
[0.066s][debug][gc,ergo ] Request concurrent cycle initiation
(requested by GC cause). GC cause: G1 Humongous Allocation
[0.066s][debug][gc,heap ] GC(0) Heap before GC invocations=0
(full 0): garbage-first heap total 20480K, used 6908K
[0x00000007fec00000, 0x0000000800000000)
[0.066s][debug][gc,heap ] GC(0) region size 1024K, 1 young
(1024K), 0 survivors (0K)
OK, so Heap Before: 1 young, 0 survivors.
Then:
[0.071s][info ][gc,heap ] GC(0) Eden regions: 1->0(9)
[0.071s][info ][gc,heap ] GC(0) Survivor regions: 0->1(2)
[0.071s][info ][gc,heap ] GC(0) Old regions: 0->0
So the next cycle will have 9 Eden Regions and 2 Survivor ones (at
least this is how I read the source code of where this is logged)
Then a GC(1) concurrent cycle happens:
[0.071s][info ][gc ] GC(1) Concurrent Cycle
And the next cycle is where I fail to understand the logging:
[0.076s][debug][gc,heap ] GC(2) Heap before GC
invocations=2 (full 0): garbage-first heap total 20480K, used 7148K
[0x00000007fec00000, 0x0000000800000000)
[0.076s][debug][gc,heap ] GC(2) region size 1024K, 2
young (2048K), 1 survivors (1024K)
How come 2 young, 1 survivors? When the previous cycle said 9 Eden, 2
Survivor.
Thank you,
Eugene.
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