On 4/5/07, Gregory Shimansky <[EMAIL PROTECTED]> wrote:
On Thursday 05 April 2007 00:48 Rana Dasgupta wrote:
> On 4/4/07, Gregory Shimansky <[EMAIL PROTECTED]> wrote:
> > On Wednesday 04 April 2007 23:33 Rana Dasgupta wrote:
> > > On 4/4/07, Mikhail Fursov <[EMAIL PROTECTED]> wrote:
> > > > On 4/4/07, Alexey Petrenko <[EMAIL PROTECTED]> wrote:
> > > > > 2007/4/4, Gregory Shimansky <[EMAIL PROTECTED]>:
> > > > > > > > I would like to see these modifications. I wonder what you've
> > > > > > > > done in
> > > > > >
> > > > > > port/src/thread/linux/apr_thread_ext.c and
> > > > > > vmcore/include/atomics.h. They contain mfence and sfence
> > > > > > instructions in inline assembly which have to be changed to
> > > > > > something else on P3.
> > >
> > > MemoryWriteBarrier() etc. should be no-ops on PIII. x86 is already
> > > strongly ordered for writes ?
> >
> > What about MemoryReadWriteBarrier()? If you know, what kind of code
> > should be used for this in P3?
>
> One of the compiler guys can confirm this. But I don't believe that
> you need to worry about any of the fence instructions fence on any of
> the PIII, PIV genuine intel procs unless you are using streaming mode
> ( SIMD ) instructions which are weakly ordered.
I actually grepped the use for MemoryReadWriteBarrier, MemoryWriteBarrier and
apr_memory_rw_barrier functions which are wrappers to mfence/sfence
instructions. They aren't used in the code which uses SSE2 in any way.
- The apr_memory_rw_barrier (executes mfence) function is used in thin locks
implementation in threading code.
- MemoryReadWriteBarrier (executes mfence) is used in
org.apache.harmony.util.concurrent natives implementation after
writing/reading int/long/object fields via JNI.
- MemoryWriteBarrier (executes sfence) is used in classloader for fast
management of classes collection and in strings pool for the same reason.
In all three cases SSE2 is not involved in any way, simply loads and stores
are done with the memory. According to you in all of those cases memory
barriers are not needed. I am just confused then why were they inserted in
those places?
I don't know the answer to this question ...unless it was intended to
cover clones etc. that don't fully support the writeback model...
--
Gregory
