Small question about JDK-8253064 and ObjectMonitor allocation
Thomas Stüfe
thomas.stuefe at gmail.com
Thu Mar 3 15:06:53 UTC 2022
Hi Dan,
sorry for being quiet so long, I'm really thankful for all the explanations.
I have one more question, if I may.
In ObjectMonitor, we carefully shepherd different groups of members into
different cache lines. But what prevents the end of one OM and the start of
the next OM to share one cacheline? If the underlying libc allocator feels
inclined to place them side by side?
I see the last member group inside an OM spilling over into the next cache
line:
```
0 ObjectMonitor* _next_om;
8 volatile intx _recursions;
16 ObjectWaiter* volatile _EntryList;
24 ObjectWaiter* volatile _cxq;
32 JavaThread* volatile _succ;
40 JavaThread* volatile _Responsible;
48 volatile int _Spinner;
52? volatile int _SpinDuration;
56? int _contentions;
64? ObjectWaiter* volatile _WaitSet;
volatile int _waiters;
volatile int _WaitSetLock;
```
Depending on whether the compiler packs 32bits into words or not, I assume
members starting at _WaitSet will start at a new cache line. The next OM
may follow immediately after that. Is that not a problem? Are we just lucky
that the natural malloc allocation granularity prevents this here? Or
should we also add padding at that point?
Thanks!
Thomas
On Wed, Feb 9, 2022 at 7:04 PM <daniel.daugherty at oracle.com> wrote:
> On 2/9/22 1:34 AM, Thomas Stüfe wrote:
>
> Hi,
>
> I wonder if I understand this correctly:
>
> - Monitors are only deflated by the MonitorDeflationThread, if I ignore
> races on inflation
>
>
> ObjectSynchronizer::deflate_idle_monitors() is called by:
>
> - the MonitorDeflationThread in its work loop
> - the VMThread at VM exit time via
> ObjectSynchronizer::do_final_audit_and_print_stats()
> - the VMThread via VM_ThreadDump::doit() when locked_monitors are
> requested in the dump
>
>
> - The MonitorDeflationThread processes them in intervals of 250ms
> (AsyncDeflationInterval),
>
>
> The MonitorDeflationThread wakes up every 250ms to check to see if there
> is work to do unless a deflation is requested in which case it wakes up
> after the notify.
>
>
> but only if there are more than 90 (MonitorUsedDeflationThreshold)
> monitors in use,
>
>
> Not a count of 90; it is a percentage. Here's the option description:
>
> product(intx, MonitorUsedDeflationThreshold, 90,
> DIAGNOSTIC, \
> "Percentage of used monitors before triggering deflation (0 is
> " \
> "off). The check is performed on GuaranteedSafepointInterval
> " \
> "or
> AsyncDeflationInterval.") \
> range(0,
> 100) \
>
> So if the used monitors count is > 90% of the "ceiling", we trigger a
> deflation. The initial ceiling estimate is based on a per-thread estimate,
> but if the in-use list has had a maximum value greater than that estimate,
> then we use the list maximum value.
>
> See src/hotspot/share/runtime/synchronizer.cpp:
> monitors_used_above_threshold().
>
>
> and only in bulks of 1mio (MonitorDeflationMax).
>
>
> Not quite in bulks of 1 million. Here's the option description:
>
> product(intx, MonitorDeflationMax, 1000000,
> DIAGNOSTIC, \
> "The maximum number of monitors to deflate, unlink and delete
> " \
> "at one time (minimum is 1024).") \
> range(1024, max_jint)
>
> so we'll deflate up to MonitorDeflationMax at one time. If there's only 1
> to deflate, then we'll do 1.
>
>
>
> Is there any upward bound preventing the creation of ObjectMonitors, since
> creation races with this one thread, and we also cap its ability to process
> more than 1mio?
>
>
> No, there is no upward bound. When the safepoint cleanup used to deflate
> idle
> monitors, that was a "natural" limit on the creation rate by virtue of the
> safepoint pausing the JavaThreads that were creating inflated
> ObjectMonitors
> while the VMThread deflated all the monitors created during the previous
> interval.
>
>
> The background of my question is that I try to get a feel for the maximum
> number of live monitors.
>
>
> There is no maximum number. If you have a badly behaving program that
> simply creates
> inflated ObjectMonitors as fast as it can, then you'll eventually bring
> the VM to its
> knees. As I've mentioned before, I've had to change the way that my
> ObjectMonitor
> stress programs work because they would create more inflated
> ObjectMonitors that
> could be dealt with by the MonitorDeflationThread.
>
>
> When running renaissance philosophers benchmark I see that the VM has,
> depending on machine speed and optimization level, > 2.5 million live
> ObjectMonitors, taking > 400MB heap space. Is this by design?
>
>
> Sounds like a perfect use for a lower MonitorUsedDeflationThreshold value
> and
> possibly a smaller AsyncDeflationInterval value, but I suspect that
> lowering
> MonitorUsedDeflationThreshold would be "enough". Of course, that's just a
> blind guess on my part.
>
> Is this behavior by the renaissance philosophers benchmark considered a
> bug or
> a feature?
>
>
> The annoying thing is that these peaks in C-heap usage can be sticky,
> depending on libc implementation. The glibc especially has poor reclaim
> behavior. Also, it is hidden in NMT under "mtInternal", which is why I
> propose to at least give it its own category (
> https://bugs.openjdk.java.net/browse/JDK-8281450).
>
>
> I think you mean:
>
> JDK-8281460 Let ObjectMonitor have its own NMT category
> https://bugs.openjdk.java.net/browse/JDK-8281460
>
> instead of:
>
> JDK-8281450 Remove unnecessary operator new and delete from
> ObjectMonitor
> https://bugs.openjdk.java.net/browse/JDK-8281450
>
> Dan
>
>
>
>
> Thanks, Thomas
>
> On Mon, Jan 31, 2022 at 11:59 PM <daniel.daugherty at oracle.com> wrote:
>
>> Greetings,
>>
>> I'm going to try and add some historical context here...
>>
>>
>> On 1/31/22 3:09 AM, Thomas Stüfe wrote:
>> > Thanks a lot for your answers, David.
>>
>> Yes David, thanks for jumping in on this thread.
>>
>>
>> > On Mon, Jan 31, 2022 at 8:35 AM David Holmes <david.holmes at oracle.com>
>> > wrote:
>> >
>> >> On 31/01/2022 3:54 pm, Thomas Stüfe wrote:
>> >>> Hi David,
>> >>>
>> >>> Thank you for the answer!
>> >>>
>> >>> On Mon, Jan 31, 2022 at 6:23 AM David Holmes <david.holmes at oracle.com
>> >>> <mailto:david.holmes at oracle.com>> wrote:
>> >>>
>> >>> Hi Thomas,
>> >>>
>> >>> On 31/01/2022 2:32 pm, Thomas Stüfe wrote:
>> >>> > Hi,
>> >>> >
>> >>> > I have a small question about a detail of JDK-8253064.
>> >>> >
>> >>> > IIUC, before this patch, the VM kept thread-local freelists of
>> >>> > pre-allocated ObjectMonitors to reduce allocation contention.
>> Now we just
>> >>> > malloc monitors right away.
>> >>> >
>> >>> > I looked through the issue and the associated PR, but could
>> find no
>> >>> > information on why this was done. Dan describes what he did
>> very well:
>> >>> >
>> https://github.com/openjdk/jdk/pull/642#issuecomment-720753946, but not
>> >>> > why.
>>
>> Thank you and I'm sorry that my words on 8253064 did not explain the why.
>>
>>
>> >>> > I assume that the complexity and memory overhead of the free
>> lists was not
>> >>> > worth it? That you found that malloc() is on our platforms
>> "uncontented"
>> >>> > enough?
>> >>>
>> >>> The issue was not about freelists and contention it was about
>> requiring
>> >>> type-stable-memory: that once a piece of memory was allocated as
>> an
>> >>> ObjectMonitor it remained forever after an ObjectMonitor. This
>> allowed
>> >>> for various race conditions in the old monitor code maintaining
>> safety.
>>
>> Erik Osterlund did have some specific reasons for getting rid of
>> type-stable-memory.
>> At least on reason was that it was complicating the work he wanted to do
>> on:
>>
>> JDK-8247281 migrate ObjectMonitor::_object to OopStorage
>> https://bugs.openjdk.java.net/browse/JDK-8247281
>>
>> He and I did work together to split this work into the various pieces that
>> were integrated separately. So TSM didn't prevent work on JDK-8247281, but
>> it did make it more difficult.
>>
>> Erik may have had other reasons for getting rid of TSM. I've added him to
>> this email thread directly so he has a better chance of seeing this query.
>>
>>
>>
>> >>> Over time that code changed substantially and the need for
>> >>> type-stable-memory for ObjectMonitors disappeared, so we finally
>> got rid
>> >>> of it and just moved to a direct allocation scheme.
>> >>>
>> >>>
>> >>> I think I understand, but I was specifically concerned with the
>> question
>> >>> of allocation contention of ObjectMonitors. That is somewhat
>> independent
>> >>> from the question of where OMs are allocated.
>> >>>
>> >>> Can it happen that lock inflation happens clustered, or does that not
>> >>> occur in reality?
>> >>>
>> >>> AFAIU the old code managed OM storage itself, used global data
>> >>> structures to do so, and guarded access with a mutex. To reduce
>> >>> contention, it used a surprisingly large thread-local freelist of 1024
>> >>> entries. This looks like contention was once a real problem.
>>
>> We need to take a step back and look at how we got where we are. There
>> are (at least) four different evolutions or seismic events to this
>> subsystem:
>>
>> 1) type-stable-memory and the global lists (block and free)
>> - JDK-5030359 "Back-end" synchronization improvements for 1.5.1 or 1.6
>> https://bugs.openjdk.java.net/browse/JDK-5030359
>> - integrated in jdk-6+50 (partial), jdk-6+53 (finished)
>>
>> - This integration introduced type-stable-memory (TSM) and global block
>> and free-lists. Dice chose to not use malloc and use TSM for a few
>> reasons:
>> - direct control of the placement of the ObjectMonitor on a
>> cache-line
>> boundary; couldn't trust malloc to keep memory properly aligned.
>> - malloc was slower than direct management of blocks of
>> ObjectMonitors.
>> - Dice did lots of benchmarking to prove the new code was faster in
>> things we cared about at the time on the platforms we cared about.
>> - This predates my joining the Runtime team so I don't have exhaustive
>> records on this work.
>>
>> 2a) per-thread free list
>> - JDK-6468954 Generic synchronization cleanups for J2SE 1.7
>> https://bugs.openjdk.java.net/browse/JDK-6468954
>> - integrated in jdk-7+12
>>
>> - This integration introduced the per-thread free list. This addition
>> was to deal with contention with allocating an ObjectMonitor from
>> the global free-list.
>> - Again, Dice did targeted benchmarking to show that adding a
>> per-thread
>> free list performed better for some specific platforms/configs.
>> - This predates my joining the Runtime team so I don't have exhaustive
>> records on this work.
>>
>> 2b) per-thread in-use list
>> - JDK-6852873 Increase in delta between application stopped time and
>> ParNew GC time over application lifetime
>> https://bugs.openjdk.java.net/browse/JDK-6852873
>> - integrated in jdk-7+99
>>
>> - This integration added the MonitorInUseLists concept and was done
>> as part of some work to deal with a safepoint performance problem.
>> - MonitorInUseLists was disabled by default.
>> - The benchmarking done here was focused on safepointing.
>> - I joined the Runtime team in Jan 2010, but I don't have exhaustive
>> records on this work.
>>
>> 2c) global in-use list
>> - JDK-6964164 +MonitorInUseLists can cause leakage of contended objects
>> https://bugs.openjdk.java.net/browse/JDK-6964164
>> - integrated in jdk-7+102
>>
>> - This integration added the global in-use list since having just a
>> global block-list and global free-list had some races that caused
>> leaks.
>> - I joined the Runtime team in Jan 2010, but I don't have exhaustive
>> records of this work.
>>
>> 2d) MonitorInUseLists on by default
>> - JDK-8149442 MonitorInUseLists should be on by default, deflate idle
>> monitors taking too long
>> https://bugs.openjdk.java.net/browse/JDK-8149442
>> - integrated in jdk-9+120
>>
>> - This default switch flip was done to improve the safepoint time
>> it takes to deflate idle monitors. The benchmarks done were again
>> safepoint focused.
>>
>> 3) async monitor deflation
>> - JDK-8153224 Monitor deflation prolong safepoints
>> https://bugs.openjdk.java.net/browse/JDK-8153224
>> - This evolution switched to async deflation and lock-free lists and
>> was very complicated.
>> -
>> https://wiki.openjdk.java.net/display/HotSpot/Async+Monitor+Deflation
>> - integrated in jdk-15+26
>>
>> - Again the focus was on reducing safepoint time. Lots of benchmarking
>> done with the perf group. We got faster safepoints, but had to
>> really
>> fight to avoid regressions in artifically contended benchmarks which
>> is where the lock-free lists stuff came from.
>>
>> 4) monitor list simplifications and getting rid of TSM
>> - JDK-8253064 monitor list simplifications and getting rid of TSM
>> https://bugs.openjdk.java.net/browse/JDK-8253064
>> - integrated in jdk-16+24
>>
>> - Not much to add here about 8253064. We wanted to get rid of TSM to
>> make other work simpler. Getting rid of TSM allowed us to switch to
>> much simpler list management logic. So we got rid of a lot of the
>> logic that we added with async monitor deflation (8153224).
>> - Lots of benchmarking done with the perf group and those
>> pre-integration
>> benchmarks showed no worrisome regressions and some speedups.
>> - Later benchmarking showed some regressions (that have since been
>> fixed)
>> and we never figured out why we had different results later
>> compared to
>> the pre-integration benchmarks. One theory is that we see different
>> results in AMD-X64 versus Intel-X64.
>>
>>
>> >> You can always create a benchmark to show contention in the monitor
>> >> inflation code. I don't recall now whether this was a real issue or a
>> >> microbenchmark issue. As the code stated:
>> >>
>> >> ObjectMonitor * ATTR ObjectSynchronizer::omAlloc (Thread * Self) {
>> >> // A large MAXPRIVATE value reduces both list lock contention
>> >> // and list coherency traffic, but also tends to increase the
>> >> // number of objectMonitors in circulation as well as the STW
>> >> // scavenge costs. As usual, we lean toward time in space-time
>> >> // tradeoffs.
>> >>
>> >> const int MAXPRIVATE = 1024 ;
>> >>
>> >> so general performance was a consideration.
>> >>
>> >>> OTOH the new code just uses malloc, which also may lock depending on
>> the
>> >>> malloc allocator internals and the used libc settings. Therefore I
>> >>> wonder whether OM allocation is still a problem, not a problem with
>> >>> real-life malloc, or maybe never really had been a problem and the old
>> >>> code was just overly cautious?
>> >> Whenever we make significant changes to a subsystem we always
>> >> investigate the performance profile of the changes. We're prepared to
>> >> accept some performance loss if we have a good improvement in code
>> >> complexity/maintainability etc, but if a significant performance issue
>> >> arose we would revisit it. See for example discussion in:
>> >>
>> >> https://bugs.openjdk.java.net/browse/JDK-8263864
>> >>
>> >> and related.
>>
>> We have not seen any performance regressions that we can attribute to
>> malloc lock contention. In fact, I had to put upper limits on my personal
>> ObjectMonitor inflation stress programs because they could allocate
>> inflated ObjectMonitors so fast that the MonitorDeflationThread could
>> not keep up. You'll notice that the deflation code now stops its
>> current loop at 1 million objects and takes a check-for-a-safepoint
>> breather. So I would say that malloc() of ObjectMonitor is not a
>> performance issue any longer. I believe Dice said that it was before
>> back in the JDK6 days, but no longer...
>>
>>
>> Hopefully some of this history stuff is useful.
>>
>> Dan
>>
>>
>>
>> >>
>> >> Cheers,
>> >> David
>> >> -----
>> >>
>> >>> Thanks, Thomas
>> >>>
>> >>>
>>
>>
>
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