Okay, I'll find time at some point to tinker with it. I'm going to wait until Mandy lands the fix to JDK-8198253, though.
Jeremy On Fri, Feb 23, 2018 at 3:44 PM, Hohensee, Paul <hohen...@amazon.com> wrote: > I am. :) > > > > The process is to file an RFE and then a CSR associated with it. You file > the CSR from the RFE page, not separately. See > > > > https://wiki.openjdk.java.net/display/csr/Main > > https://wiki.openjdk.java.net/display/csr/Fields+of+a+CSR+Request > > https://wiki.openjdk.java.net/display/csr/CSR+FAQs > > > > You need a review of the CSR by a domain expert before you can finalize it > for committee review. If you go with com.sun.management versions of > ThreadInfo and ThreadMXBean methods, there’s no change in the > java.lang.management specification, which is important because the j.l.m > interface must be supported by all implementations, which is a higher bar > than for c.s.m changes. > > > > Paul > > > > *From: *Jeremy Manson <jeremyman...@google.com> > *Date: *Friday, February 23, 2018 at 3:12 PM > *To: *"Hohensee, Paul" <hohen...@amazon.com> > *Cc: *David Holmes <david.hol...@oracle.com>, "serviceability-dev@openjdk. > java.net" <serviceability-dev@openjdk.java.net>, " > loom-...@openjdk.java.net" <loom-...@openjdk.java.net>, John Rose < > john.r.r...@oracle.com> > > *Subject: *Re: Thread Native ID Access > > > > Thanks, Paul. This is approximately what I was envisioning when Mandy and > David said they didn't really want to expose it in java.lang.Thread. > > > > If folks are generally amenable to something like this, I can probably > find the spare time to cobble it together. > > > > Jeremy > > > > On Fri, Feb 23, 2018 at 1:07 PM, Hohensee, Paul <hohen...@amazon.com> > wrote: > > Here’s a concrete proposal on which people may want to comment. > > > > We could add a method to ThreadInfo, vis., > > > > long getPlatformThreadId() > > > > Returns the platform-specific thread ID of the thread associated with this > ThreadInfo. The ID has no meaning to the Java runtime and is not guaranteed > to have any meaning at all. > > I.e., it may be a constant value for all threads, or a native address, and > may not be the same across multiple invocations. > > > > Returns: > > > > the platform-specific ID of the associated thread. > > > > For a fiber, a reasonable value would be the OS thread id for the OS > thread that’s running the fiber at ThreadInfo construction time. If, say, > the platform used a string or some other larger-than-long value, it could > be a native address or some other value that mapped to one. > > > > If we don’t want to add a new method to ThreadInfo, we could define a > com.sun.management.ThreadInfo to go with com.sun.management.ThreadMXBean. > > > > Parenthetically, to me a fiber is enough different from a thread to > warrant a separate MXBean. I.e., a thread is a species of fiber, but not > vica versa. So, we shouldn’t gate changes to ThreadMXBean on having a > thorough understanding of how to support fibers in JMX. > > > > We could also add to com.sun.management.ThreadMXBean the methods > getThreadId(long id), getThreadId(long [] ids), getPlatformThreadId(long > id), and getPlatformThreadId(long [] ids). These presumably would be fast > compared to creating ThreadInfo(s). > > > > We don’t want to add Thread.getPlatformId(), so the way to get the current > thread’s platform id would be > > > > getThreadInfo( Thread.currentThread().getId() ).getPlatformThreadId() > > > > which is cumbersome and slow. Instead, by analogy to > ThreadMXBean.getCurrentThreadCpuTime(), > we could add to com.sun.management.ThreadMXBean the methods > getCurrentThreadId() and getCurrentPlatformThreadId(). > > > > All this may seem somewhat overkill, but it’s no more so than how we > currently create ThreadInfo objects and the various ThreadCpu/UserTime > method definitions. > > > > Thanks, > > > > Paul > > > > *From: *serviceability-dev <serviceability-dev-boun...@openjdk.java.net> > on behalf of Jeremy Manson <jeremyman...@google.com> > *Date: *Thursday, February 22, 2018 at 9:42 PM > *To: *David Holmes <david.hol...@oracle.com> > *Cc: *"serviceability-dev@openjdk.java.net" <serviceability-dev@openjdk. > java.net>, "loom-...@openjdk.java.net" <loom-...@openjdk.java.net>, John > Rose <john.r.r...@oracle.com> > *Subject: *Re: Thread Native ID Access > > > > I think we're all pretty much agreed that fibers imply native tids can > change. I think that, as long as we document that, we're fine. > > > > An API where there is probably a worse implementation problem is > ThreadMXBean#getThreadCpuTime > <https://docs.oracle.com/javase/9/docs/api/java/lang/management/ThreadMXBean.html#getThreadCpuTime-long-> > . Right now, the implementation assumes a 1:1 mapping between OS thread > and Java thread. Removing that assumption implies you have to do something > like track cpu time every time the scheduler switches you out, which is > going to make your switches expensive. > > > > Jeremy > > > > On Thu, Feb 22, 2018 at 9:16 PM, David Holmes <david.hol...@oracle.com> > wrote: > > On 23/02/2018 3:04 PM, John Rose wrote: > > On Feb 22, 2018, at 8:49 PM, David Holmes <david.hol...@oracle.com > <mailto:david.hol...@oracle.com>> wrote: > > > I don't think this request has any impact on Fibers at all. At any given > time a piece of executing Java code is executing on a current Thread, and > that current Thread must be running on a native thread (regardless of > mapping) and the native thread has an id. The only use for exposing such an > id is to allow you to correlate information obtained from inside the VM, > with information observed outside, possibly by external tools. > > > We may or may not bind fiber identity to the legacy java.lang.Thread type. > > > I'm not assuming that exposing native thread ids implies anything about > the binding between those entities and anything higher-level. I have no > issue with a Fiber/Continuation/Task/whatever running on different > java.lang.Threads over its lifetime. For that matter I don't think it > matters if java.lang.Threads could execute on different native threads > (they don't but they could). > > David > ----- > > > > This affects the question of whether the following code can return false: > > boolean testThreadShift(Runnable r) { > Thread t0 = Thread.current(); > r.run(); > Thread t1 = Thread.current(); > return t0 == t1; > } > > This method can return false if (a) Thread.current() reports the identity > of the OS thread (not the fiber currently running), and (b) the runnable > includes a blocking operation which causes the current fiber to reschedule > on a different OS thread before it returns. > > Having this method return false is, perhaps, surprising enough to cause > us to inject fiber identity into java.lang.Thread, so that a > java.lang.Thread > identity is 1-1 with a fiber, rather than an OS "dinosaur" thread. (You > can > only pick one!) The alternative is to keep a 1-1 identity between OS > threads > and java.lang.Thread instances, making a new type which carries fiber > identity that multiplexes in a scheduler-dependent way over OS threads. > > It's an interesting trade-off, with many compatibility concerns. > I think we are leaning toward injecting fiber identity into Thread, > although this makes me shudder to think of the overheads we > will buy with this one move (ThreadLocal hash tables). > > So, now if you reify the OS thread identity, you could have the same > issue again. Can this routine return false? > > boolean testNTIDShift(Runnable r) { > long t0 = NativeThreadID.current(); > r.run(); > long t1 = NativeThreadID.current(); > return t0 == t1; > } > > I think, if you are digging down all the way to the native thread ID, > this routine *must* be allowed to return false under a Loom JVM. > There's no credible way to assign unique int64_t IDs to fiber objects > in the Java heap, as if they were OS objects. > > So my question is, do the proposed use cases for NTID allow > for testNTIDShift to return false? If not, then it's pretty easy to say > that we don't want to supply NativeThreadID.current() for general > use, just to people implementing schedulers. > > — John > > > > >