[concurrency-interest] padding in Exchanger

Ruslan Cheremin cheremin at gmail.com
Tue Jan 17 13:06:07 EST 2012


I'm sure the same things was said by people against GC few decades ago :)

2012/1/17 Vitaly Davidovich <vitalyd at gmail.com>:
> 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 | Consulting Member of Technical Staff | 602.333.9091
>> Oracle PSR Engineering | 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>:
>>
>> 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> wrote:
>>
>> 2012/1/17 Vitaly Davidovich <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> 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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