RFR: 8231666: ThreadIdTable::grow() invokes invalid thread transition
David Holmes
david.holmes at oracle.com
Fri Oct 4 04:15:30 UTC 2019
Hi Daniil,
On 4/10/2019 1:38 pm, Daniil Titov wrote:
> Hi David and Robbin,
>
> Please review a new version of the fix that makes the service thread responsible for the thread table growth.
>
> Webrev: http://cr.openjdk.java.net/~dtitov/8231666/webrev.02/
> Bug: https://bugs.openjdk.java.net/browse/JDK-8231666
I don't think you need to repeat the load factor check here:
void ThreadIdTable::do_concurrent_work(JavaThread* jt) {
assert(_is_initialized, "Thread table is not initialized");
_has_work = false;
double load_factor = get_load_factor();
log_debug(thread, table)("Concurrent work, load factor: %g",
load_factor);
if (load_factor > PREF_AVG_LIST_LEN &&
!_local_table->is_max_size_reached()) {
grow(jt);
}
}
as we will only execute this code if the load factor was seen to be too
high.
You might also want to put the max size check in the
check_concurrent_work code:
+ // Resize if we have more items than preferred load factor
+ if ( load_factor > PREF_AVG_LIST_LEN &&
!_local_table->is_max_size_reached()) {
so that we don't keep waking up the service thread for nothing if the
table gets full.
Thanks,
David
-----
> Testing: Mach5 tier1, tier2, and tier3 tests successfully passed.
>
> Thank you!
>
> Best regards,
> Daniil
>
> On 10/2/19, 3:26 PM, "David Holmes" <david.holmes at oracle.com> wrote:
>
> Hi Daniil,
>
> On 3/10/2019 2:21 am, Daniil Titov wrote:
> > Hi David and Robbin,
> >
> > Could we consider making the ServiceThread responsible for the ThreadIdTable resizing in the similar way how
> > it works for StringTable and ResolvedMethodTable, rather than having ThreadIdTable::add() method calling ThreadIdTable::grow()?
> > As I understand It should solve the current issue and address the concern that the doing the resizing could be a relatively long and
> > doing it without polling for safepoints or while the holding Threads_lock is not desirable.
>
> I originally rejected copying that part of the code from the other
> tables as it seems to introduce unnecessary complexity. Having a
> separate thread trying to grow the table when it could be concurrently
> having threads added and removed seems like it could introduce hard to
> diagnose performance pathologies. It also adds what we know to be a
> potentially long running action to the workload of the service thread,
> which means it may also impact the other tasks the service thread is
> doing, thus potentially introducing even more hard to diagnose
> performance pathologies.
>
> So this change does concern me. But go ahead and trial it.
>
> Thanks,
> David
>
>
> > Thank you,
> > Daniil
> >
> >
> > On 10/2/19, 6:25 AM, "David Holmes" <david.holmes at oracle.com> wrote:
> >
> > Hi Robbin,
> >
> > On 2/10/2019 7:58 pm, Robbin Ehn wrote:
> > > Hi David,
> > >
> > >> What if the table is full and must be grown?
> > >
> > > The table uses chaining, it just means load factor tip over what is
> > > considered a good backing array size.
> >
> > Coleen raised a good question in a separate discussion, which made me
> > realize that once the table has been initially populated all subsequent
> > additions, and hence all subsequent calls to grow() always happen with
> > the Threads_lock held. So we can't just defer the grow().
> >
> > >> That aside, I'd like to know how expensive it is to grow this table.
> > >> What are we talking about here?
> > >
> > > We use global counter which on write_synchronize must scan all
> > > threads to make sure they have seen the update (there some
> > > optimization to avoid it if there is no readers at all). Since this
> > > table contains the threads, we get double penalized, for each new
> > > thread the synchronization cost increase AND the number of items.
> > >
> > > With concurrent reads you still need many thousands of threads, but
> > > I think I saw someone mentioning 100k threads, assuming concurrent
> > > queries the resize can take hundreds of ms to finish. Note that reads
> > > and inserts still in operate roughly at the same speed while
> > > resizing. So a longer resize is only problematic if we do not
> > > respect safepoints.
> > I think if anything were capable of running 100K threads we would be
> > hitting far worse scalability bottlenecks than this. But this does seem
> > problematic.
> >
> > Thanks,
> > David
> > -----
> >
> > > Thanks, Robbin
> > >
> > >>
> > >> David
> > >>
> > >>> /Robbin
> > >>>
> > >>> On 2019-10-02 08:46, David Holmes wrote:
> > >>>> Hi Daniil,
> > >>>>
> > >>>> On 2/10/2019 4:13 pm, Daniil Titov wrote:
> > >>>>> Please review a change that fixes the issue. The problem here is
> > >>>>> that that the thread is added to the ThreadIdTable (introduced in
> > >>>>> [3]) while the Threads_lock is held by
> > >>>>> JVM_StartThread. When new thread is added to the thread table the
> > >>>>> table checks if its load factor is greater than required and if so
> > >>>>> it grows itself while polling for safepoints.
> > >>>>> After changes [4] an attempt to block the thread while holding the
> > >>>>> Threads_lock results in assertion in
> > >>>>> Thread::check_possible_safepoint().
> > >>>>>
> > >>>>> The fix proposed by David Holmes ( thank you, David!) is to skip
> > >>>>> the ThreadBlockInVM inside ThreadIdTable::grow() method if the
> > >>>>> current thread owns the Threads_lock.
> > >>>>
> > >>>> Sorry but looking at the fix in context now I think it would be
> > >>>> better to do this:
> > >>>>
> > >>>> while (gt.do_task(jt)) {
> > >>>> if (Threads_lock->owner() == jt) {
> > >>>> gt.pause(jt);
> > >>>> ThreadBlockInVM tbivm(jt);
> > >>>> gt.cont(jt);
> > >>>> }
> > >>>> }
> > >>>>
> > >>>> This way we don't waste time with the pause/cont when there's no
> > >>>> safepoint pause going to happen - and the owner() check is quicker
> > >>>> than owned_by_self(). That partially addresses a general concern I
> > >>>> have about how long it may take to grow the table, as we are
> > >>>> deferring safepoints until it is complete in this JVM_StartThread
> > >>>> usecase.
> > >>>>
> > >>>> In the test you don't need all of:
> > >>>>
> > >>>> 32 * @run clean ThreadStartTest
> > >>>> 33 * @run build ThreadStartTest
> > >>>> 34 * @run main ThreadStartTest
> > >>>>
> > >>>> just the last @run suffices to build and run the test.
> > >>>>
> > >>>> Thanks,
> > >>>> David
> > >>>> -----
> > >>>>
> > >>>>> Testing : Mach 5 tier1 and tier2 completed successfully, tier3 is
> > >>>>> in progress.
> > >>>>>
> > >>>>> [1] Webrev: http://cr.openjdk.java.net/~dtitov/8231666/webrev.01/
> > >>>>> [2] Bug: https://bugs.openjdk.java.net/browse/JDK-8231666
> > >>>>> [3] https://bugs.openjdk.java.net/browse/JDK-8185005
> > >>>>> [4] https://bugs.openjdk.java.net/browse/JDK-8184732
> > >>>>>
> > >>>>> Best regards,
> > >>>>> Danill
> > >>>>>
> > >>>>>
> >
> >
> >
>
>
>
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