RFD: 8252768: Fast, asynchronous heap dumps

[Thomas Stüfe]

On Thu, Sep 3, 2020, 21:27 Laurence Cable <larry.cable at oracle.com> wrote:

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> On 9/3/20 12:25 PM, Thomas Stüfe wrote:
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> On Thu, Sep 3, 2020, 21:07 Laurence Cable <larry.cable at oracle.com> wrote:
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>> On 9/3/20 9:03 AM, Volker Simonis wrote:
>> > Hi,
>> >
>> > I'd like to get your opinion on a POC I've done in order to speed up
>> > heap dumps on Linux:
>> >
>> > https://bugs.openjdk.java.net/browse/JDK-8252768
>> > http://cr.openjdk.java.net/~simonis/webrevs/2020/8252768/
>> >
>> > Currently, heap dumps can be taken by the SA tools from a frozen
>> > process or core file or directly from a running process with jcmd,
>> > jconsole & JMX, jmap, etc. If the heap of a running process is dumped,
>> > this happens at a safepoint (see VM_HeapDumper). Because the time to
>> > produce a heap dump is roughly proportional to the size and fill ratio
>> > of the heap, this leads to safepoint times which can range from ~100ms
>> > for a 100mb heap to ~1s for a 1gb heap up to 15s and more for a 8gb
>> > heap (measured on my Core i7 laptop with SSD).
>> >
>> > One possibility to decrease the safepoint time is to offload the
>> > dumping work to an asynchronous process. On Linux (and probably any
>> > other OS which supports fork()) this can be achieved by forking and
>> > offloading the heap dumping to the child process. Forking still needs
>> > to happen at a safepoint, but forking is considerably faster compared
>> > to the dumping process itself. The fork performance is still
>> > proportional to the size of the original Java process because although
>> > fork won't copy any memory pages, the kernel still needs to duplicate
>> > the page table entries of the process.
>>
>
> curious what is the. behavior of the parent/target JVM process "after"
>> it executes the fork() at the safepoint? i.e what does it do next?
>>
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> It just continues its life. It will periodically try to reap the child
> process, but apart from that it will just run on.
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> so then the state of the (parent's) heap may change "under" the dumping
> child?
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> what am I missing?
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When forking, the child process gets a *copy* of the parents address space.
That is if one were to implement fork() naively. Since copying the parent
address space would be prohibitively expensive, especially for parent
processes with a large footprint, modern OSes do a copy-on-write.

The child gets a copy of the address space of the parent at the time the
fork was done. As long as the child only reads a page, and the page has not
been modified by the parent, this is physically the same memory. If either
parent or child writes to the page it gets really duplicated.

Effectively, Volker uses the OS to get a frozen snapshot of the java heap
at fork time.


> > Linux uses a “copy-on-write” technique for the creation of a forked
>> > child process. This means that right after creation, the child process
>> > will have exactly the same memory image like its parent process. But
>> > at the same time, the child process won’t use any additional physical
>> > memory, as long as it doesn’t change (i.e. writes into) its memory.
>> > Since heap dumping only reads the child process's memory and then
>> > exits immediately, this technique can be applied even if the Java
>> > process already uses almost the whole free physical memory.
>> >
>> > The POC I've created (see
>> > http://cr.openjdk.java.net/~simonis/webrevs/2020/8252768/) decreases
>> > the aforementioned ~100ms, ~1s and 15s for a 100mb, 1gb and 8gb heap
>> > to ~3ms, ~15ms and ~60ms on my laptop which I think is significant.
>> > You can try it out by using the new "-async" or "-async=true" option
>> > of the "GC.heap_dump" jcmd command.
>> >
>> > Of course this change will require a CSR for the additional jcmd
>> > GC.heap_dump "-async" option which I'll be happy to create if there's
>> > any interest in this enhancement. Also, logging in the child process
>> > might potentially interfere with logging in the parent VM and probably
>> > will have to be removed in the final version, but I've left it in for
>> > now to better illustrate what's happening. Finally, we can't output
>> > the size of the created dump any more if we are using asynchronous
>> > dumping but from my point of view that's not such a big problem. Apart
>> > from that, the POC works surprisingly well :)
>> >
>> > Please let me know what you think and if there's something I've
>> overlooked?
>> >
>> > Best regards,
>> > Volker
>> >
>> > PS: by the way, asynchronous dumping combines just fine with
>> > compressed dumps. So you can easily use "GC.heap_dump -async=true
>> > -gz=6"
>>
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