(L) Prelim RFR: 8132510: Replace ThreadLocalStorage with compiler/language-based thread-local variables
David Holmes
david.holmes at oracle.com
Wed Nov 4 04:08:48 UTC 2015
Hi Thomas,
On 4/11/2015 12:14 AM, Thomas Stüfe wrote:
> On Mon, Nov 2, 2015 at 9:01 PM, David Holmes <david.holmes at oracle.com
> <mailto:david.holmes at oracle.com>> wrote:
> On 2/11/2015 9:20 PM, Thomas Stüfe wrote:
> some small changes are needed to make this build and run on AIX. I
> attached a patch file with the needed additions.
>
> I also checked Linux ppc64, seems to work fine.
Excellent! Thank you!
> I did not run any extensive tests on AIX, so I cannot say for
> sure if
> this is stable. We (SAP) also may face some problems later when
> we port
> this to HP-UX, because there, shared libraries using __thread
> cannot be loaded dynamically.
>
> Ouch!
>
> So, I admit to some small worries, beside the issue with memory
> leaks on
> older glibc versions. For me, this feels like something which needs
> tight compiler/thread library support from the OS, so it makes us
> vulnerable to running on older systems (older glibc) or building
> with
> outdated compilers. Therefore it would be nice to have a simple
> way to re-add the pthread-based TLS implementation if needed.
>
> I can't see how to do that without keeping all the existing layers
> of code - even though they would be no-ops on all the platforms that
> support the compiler-based TLS. Basically just extend what I did for
> Solaris to the other platforms.
>
> I took a closer look and I now I worry less. I am confident that in case
> our old platforms experience problemswith __thread, we can reintroduce
> TLS without too many changes.
>
> Just as a test, I changed the AIX implementation from using __thread
> back to pthread tls just by changing implementations for
> Thread::current(), Thread::initialize_thread_current() and
> Thead::clear_thread_current() in thread.cpp. Works fine as expected. Of
> course this was just a hack, but if we need to go back to pthread tls
> for AIX or any platform, I think it can be done in a simpler way than
> before and still be clean.
Thanks for looking into this in detail! Yes I've been thinking about
this too, and I think three of four simple hooks will allow the basic
pthread-TLS mechanism to be reinstated, in shared code, but without any
of the per-platform fancy caching schemes. There will be a single
threadLocalStorage.cpp file in a platform specific directory; and of
course MacroAssembler::get_thread may need to be os/cpu specific. I will
look further into this, but may defer its implementation to a follow up
issue.
> Not terribly important, but I would prefer if
> Thread::initialize_thread_current() and Thead::clear_thread_current()
> were not exposed from Thread at all or at least as private as possible.
> Thread::initialize_thread_current() is called from the OS layer, but
> Thead::clear_thread_current() is only called from within thread.cpp
> itself and could be kept at file scope.
As you note the initialize function has to be exposed as it is needed in
the OS thread startup code. But I can make the clear function private.
> Apart from that, I like the patch and think the simplification
> is good and worth the effort.
>
> Even if you can't easily add back the pthread-based TLS if needed?
>
> I think we can, if needed.
Ok.
> It is unfortunate that hotspot may still be shackled to the past
> that way - we killed off hotspot-express (in part) to remove those
> shackles and allow us to modernize the codebase.
>
> Thanks,
> David
>
>
> One question about your changes:
>
> Before, Thread::current() would assert instead of returning NULL if
> called before Thread::initialize_thread_current() or after
> Thead::clear_thread_current() . Now, we just return NULL. Is this intended?
Ah great question ... so before we have a mix of calls to:
- Thread::current() (asserts on NULL as does JavaThread::current)
- ThreadLocalStorage::thread() (can return NULL)
- ThreadLocalStorage::get_thread_slow() (can return NULL)
and now we only have Thread::current() which means we have to allow
returning NULL because it can be intentionally called when a thread is
not attached. That means we won't directly catch calls to
Thread::current() from code that doesn't realize it is calling it "too
soon" - though there do exist numerous assertions in the callers of
Thread::current() that check the result is not NULL.
I could add the assert to Thread::current() and also add
Thread::current_or_null() to be used by code that needs to use it to
check for attachment (ie JNI code). I'd also have to examine all the
changed ThreadLocalStorage::thread/get_thread_slow call-sites to see if
any of those legitimately expect the thread may not be attached.
What do you think?
I also need to look at the location of Thread::current in the .hpp file
rather than .inline.hpp and reconcile that with comments regarding the
noinline version (which is only used in g1HotCardCache.hpp).
Thanks,
David
> Regards, Thomas
>
> Kind Regards, Thomas
>
>
>
>
>
>
> On Mon, Nov 2, 2015 at 7:40 AM, David Holmes
> <david.holmes at oracle.com <mailto:david.holmes at oracle.com>
> <mailto:david.holmes at oracle.com
> <mailto:david.holmes at oracle.com>>> wrote:
>
> bug: https://bugs.openjdk.java.net/browse/JDK-8132510
>
> Open webrev:
> http://cr.openjdk.java.net/~dholmes/8132510/webrev.v2/
>
> A simple (in principle) but wide-ranging change which
> should appeal
> to our Code Deletion Engineer's. We implement Thread::current()
> using a compiler/language-based thread-local variable eg:
>
>
> static __thread Thread *_thr_current;
>
> inline Thread* Thread::current() {
> return _thr_current;
> }
>
> with an appropriate setter of course. By doing this we can
> completely remove the platform-specific ThreadLocalStorage
> implementations, and the associated
> os::thread_local_storage* calls,
> plus all the uses of ThreadLocalStorage::thread() and
> ThreadLocalStorage::get_thread_slow(). This extends the
> previous
> work done on Solaris to implement
> ThreadLocalStorage::thread() using
> compiler-based thread-locals.
>
> We can also consolidate nearly all the os_cpu versions of
> MacroAssembler::get_thread on x86 into one cpu specific one ( a
> special variant is still needed for 32-bit Windows).
>
> As a result of this change we have further potential cleanups:
> - all the src/os/<os>/vm/thread_<os>.inline.hpp files are now
> completely empty and could also be removed
> - the MINIMIZE_RAM_USAGE define (which avoids use of the linux
> sp-map "cache" on 32-bit) now has no affect and so could be
> completely removed from the build system
>
> I plan to do the MINIMIZE_RAM_USAGE removal as a follow up
> CR, but
> could add the removal of the "inline" files to this CR if
> people
> think it worth removing them.
>
> I have one missing piece on Aarch64 - I need to change
> MacroAssembler::get_thread to simply call Thread::current()
> as on
> other platforms, but I don't know how to write that. I would
> appreciate it if someone could give me the right code for that.
>
> I would also appreciate comments/testing by the AIX and
> PPC64 folk
> as well.
>
> A concern about memory-leaks had previously been raised, but
> experiments using simple C code on linux 86 and Solaris
> showed no
> issues. Also note that Aarch64 already uses this kind of
> thread-local.
>
> Thanks,
> David
>
>
>
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