The fast-path overhead in this polling scheme (calling code at a specific
memory location and returning from it, relying on remapping of the page the
code us in to change its behavior from do-nothing to take-safepoint) is
much higher than the currently-popular polling schemes of loading from a
HotSpot used to actually safepoint by patching the running code of threads,
at some point ahead of where you suspended them. The notion was that this
lets you completely avoid any polling instructions and Keeps the fast path
as fast as possible. HotSpot gave up on doing this almost 20 years
ago
> On 29 May 2020, at 17:46, Steven Stewart-Gallus
> wrote:
>
> Okay I have an idea.
> I can't shake the idea you could do fun tricks with thread local executable
> pages.
>
> The theoretically fastest way of safepoint polling is inserting a trap
> instruction.
Under what basis are you making
Behold!
I don't think this actually safely works though on x86 at least.
Pretty sure they use the virtual address for instruction caching and
debuggers have to do synchronisation when modifying from a different
address space.
oh well.
On Friday, May 29, 2020 at 9:46:38 AM UTC-7, Steven Stewart
Okay I have an idea.
I can't shake the idea you could do fun tricks with thread local executable
pages.
The theoretically fastest way of safepoint polling is inserting a trap
instruction. But icache overheads dominate. If the icache is based on physical
addresses and not virtual ones then it sh
Hi
What I guess I sort of mean is that I'm looking for more papers like this
one
https://dl.acm.org/doi/10.1145/2887746.2754187 . I guess I did not explain
myself well.
I'm well aware of the theory behind choosing a simple read but the key word
is "empirically chosen". Are there public benchmarks
This is an evolving and ever-explored field...
The "current" (and in typically used in production 8 and 11) versions of
OpenJDK and HotSpot performs safepoint as an all-or-nothing, Stop-The-World
(STW) event. Since the frequency of STW pauses will generally tend to be
low (for obvious reasons,
In Hotspot there's also JEP 312 - http://openjdk.java.net/jeps/312 -
as of Java 10 which might be useful to you, depending on your use
case.
Anyone familiar enough with J9 to comment on how that works? I see to
remember from talking to Gil that the approach Zing takes is also
interesting, but that
There’s a couple of overlapping questions here. I hope I can answer them, if
not necessarily the right order.
Reads are used rather than writes because reads don’t incur cross-CPU cache
invalidations. If threads were writing to a page, then cache invalidation
traffic would be sent between CPUs,
Hi
As I understand it most VMs poll for safepoints by using memory management
tricks to page fault polling threads. A page is set aside to read from and
whenever a safepoint is desired the page is set to be unreadable.
But can't a number of other hardware traps be used instead
https://wiki.osde
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