Performance analysis of ZGC load barrier for oop arraycopy

[Bhavana Kilambi]

Hello,

My apologies for sending out an attachment in my previous email, when I 
was supposed to provide a link for the same. Will keep it mind going 
further. The document is copied here - 
http://cr.openjdk.java.net/~smonteith/BK/ZGC%20arraycopy%20improvements.pdf


Thank you,

Bhavana


On 4/19/22 13:54, Bhavana Kilambi wrote:
> Hello,
>
> I would like to share some analysis work that I've done on the load 
> barrier on arraycopy in ZGC.
>
> This PR - https://github.com/openjdk/jdk/pull/6594 introduced stress 
> tests for arraycopy where ObjectArrayCopy ended up in a timeout 
> failure on a Windows-x64 machine with ZGC. This prompted us to perform 
> some performance analysis/testing to understand the behaviour of ZGC 
> and other GCs for arraycopy of objects.
>
> Used a simple JMH testcase with a call to System.arraycopy() to copy 
> an entire array of 1024 object references to another array and ran it 
> with the six available garbage collectors in OpenJDK 17,18 (Epsilon, 
> G1, Z, Shenandoah, Serial, Parallel) on Neoverse N1 and Skylake 
> systems. The performance of all the GCs except ZGC was more or less 
> similar but the runtime with ZGC was ~8x that of G1GC (taken as 
> representative of the rest of the GCs) on the N1 system and ~10x on 
> the Skylake system (with OpenJDK17).
>
> The actual hot loop is this -
>
>
> inline void ZBarrier::load_barrier_on_oop_array(volatile oop* p, 
> size_t length) {
>
> for (volatile const oop* const end = p + length; p < end; p++) {
>
> load_barrier_on_oop_field(p);
>
> }
>
> }
>
> Tried to optimize this loop by unrolling it and hoisting the load of 
> the bad_mask out of this loop and these changes showed significant 
> improvement in the runtimes of the JMH testcase and for a couple of 
> real world workloads.
>
> A detailed analysis report with comparative analysis between GCs, 
> profiles and code changes are present in the attached document.
>
> Would like to know your thoughts on this.
>
>
> Thank you,
>
> Bhavana