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.hol...@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.hol...@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
> >>>>>
> >>>>>
>
>
>
