[concurrency-interest] padding in Exchanger

Vitaly Davidovich vitalyd at gmail.com
Tue Jan 17 13:03:29 EST 2012


Doing dynamic re-layout and contention adjustment seems nice in thought but
how practical is that? I can't see how that would be cheap enough where
it's worth the cost. What if the app goes through phases of contention?
Initially high but then no sharing - would the bloated object layout be
worth it at that point? Seems like this is a place where explicit developer
instructions is better than heuristics with potentially expensive
consequences.

Sent from my phone
On Jan 17, 2012 12:44 PM, "Nathan Reynolds" <nathan.reynolds at oracle.com>
wrote:

>  Assuming that the JVM can optimize for true and false sharing (and that
> is a big assumption at the moment), then you can focus your time on writing
> useful code.  Furthermore, optimizing for true and false sharing will never
> be able to fix actual data contention.  We still need clever ways of
> sharing data without bottlenecks.
>
> Nathan Reynolds<http://psr.us.oracle.com/wiki/index.php/User:Nathan_Reynolds>| Consulting Member of Technical Staff |
> 602.333.9091
> Oracle PSR Engineering <http://psr.us.oracle.com/> | Server Technology
>
> On 1/17/2012 10:29 AM, Ruslan Cheremin wrote:
>
> You made my day. Few months ago I was dreaming (in my blog) about
> complexity of false sharing prevention with padding. And come to two
> options, one better another. First one was @PreventFalseSharing
> annotation, next was atomatically contention detection and relocation
> of contended objects by JIT. Readers of my blog soon pointed me to
> @Contended annotation. And now you telling the second -- the best --
> option is also being explored!
>
> Just want to ask -- if all good things will be done by JIT -- what I
> will be paid for?
>
> 2012/1/17 Nathan Reynolds <nathan.reynolds at oracle.com> <nathan.reynolds at oracle.com>:
>
>  It would be nice if the processor could effectively tell the JVM that false
> sharing is happening.  It would be nice if the JVM could respond by moving
> objects within the heap or fields with the class to avoid false sharing.
> Thus, we don't have to pad or worry about placing @Contended or other
> attributes into the class.
>
> Intel was looking into to optimizing for true and false sharing.  They had a
> prototype that worked but required restarting the JVM.  Oracle was looking
> into dynamically relayout fields in objects.  I haven't heard anything from
> either group for a while...  I haven't asked either.  *IF* a solution
> becomes available, then it will be a while.  This is a very difficult thing
> to do.
>
> Nathan Reynolds | Consulting Member of Technical Staff | 602.333.9091
> Oracle PSR Engineering | Server Technology
>
> On 1/17/2012 9:35 AM, Vitaly Davidovich wrote:
>
> OK I see what you mean now.  I imagine @Contended will be used with fields
> rather than classes so when the JVM lays out an instance of the class I
> assume it will do two-sided padding on the contended field if required or if
> natural layout is such that prior fields already fill up a cache line then
> only one sided is needed.
>
> Sent from my phone
>
> On Jan 17, 2012 11:27 AM, "Ruslan Cheremin" <cheremin at gmail.com> <cheremin at gmail.com> wrote:
>
>
> 2012/1/17 Vitaly Davidovich <vitalyd at gmail.com> <vitalyd at gmail.com>:
>
>  I think it's semantics - if you sometimes allocate with 64/128 byte
> alignment then if your object is smaller than 64/128 the rest of the
> space
> is effectively padding.
>
>
> Agree. But in case of alignment you lose sense of "one-side" or "two
> side" padding -- you do not need "two side padding", you just make
> sure object align on cache line boundary.
>
> Actually I was asked is my understanding of how @Contended supposed to
> be implemented is right?
>
>
>  Or are you saying you want an @Alignment annotation
> instead so it's more general? What other uses of custom alignment do you
> envision? Java is too high-level  and the underlying hardware/platform
> too
> abstracted away for a general purpose custom alignment hint, IMHO.
>
>
> No, I do not want such ugly thing to happen with java! It's enough C
> for such things...
>
>
>
>  Sent from my phone
>
> On Jan 17, 2012 10:56 AM, "Ruslan Cheremin" <cheremin at gmail.com> <cheremin at gmail.com> wrote:
>
>   Yes. As a practical matter though, until an @Contended attribute
> or something like it is supported across JVMS (see list archives for
> discussion), you cannot arrange reliable two-sided padding
> for objects with mixed field types (ints, longs, refs that may be
> either 32 or 64 bits, etc), so one-sided is the best you can do.
>
>
> By the way -- I was not thinking about @Contended as "make padding for
> me". It seems for me like padding is only dirty hack, since nothing
> better available. If I would control memory allocation (like JVM does)
> I just can allocate @Contended objects on 64 (128... etc) bytes
> boundary. I do not have to "pad" them -- nor both, nor one side. And I
> suppose @Contended implementation to do exactly this -- "use special
> allocator for objects of that type, which allocate them on cache line
> boundary"
>
> Am I wrong here?
>
>
>
>
> -Doug
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