Re: [Gems-users] Tourmaline and scheduling

[Dan Gibson <degibson@xxxxxxxx>]

Matthew,
We don't modify Solaris's scheduling policy -- we do usually bind transactional threads to processors in our workloads, however. This tends to promote the gang scheduling and concurrency, though admittedly, it is not always 100% successful.

You might also look out for low transactional duty cycles overall (I'm not very familiar with STAMP), short transaction sizes, and transactions that conflict very very rarely (probably a common case in a well-tuned application. Again, I am not very familiar with STAMP).

The other thing to look for is the possibility that LogTM's released implementation will blindly stall on conflicts, perhaps for a long long time, with no regards to potential performance pathologies. I'd look at the above issues first, however.

Regards,
Dan

Matthew James Horsnell wrote:

Dan/Derek,

Thanks for your reply. I guess I probably didn't phrase my question well
enough. I appreciate that no performance results in LogTM were presented
using the tourmaline module. However, my question is equally applicable
to the Gems-Ruby modules; do you in any way alter the OS thread
scheduling policy in Solaris?. Using tourmaline, and although the ruby
modules will add timing delays, I see very infrequent overlapping of
transactional threads, mainly do to the influence that the OS scheduling
policy appears to have on the way the transactional threads are
scheduled. For example on a 4 processor system, I rarely see all
processors executing transactionally, and for long periods of time, only
one processor is executing a transactional thread.

Perhaps I am misunderstanding something, but currently with the OS
scheduling the transactional threads (in my case running the STAMP
kmeans benchmark) I hardly see any concurrency, not due to aborted
transactions but rather because the operating system is rarely choosing
to scheduling multiple transactional threads concurrently.

Any more information would be gratefully received,

Matt

Derek Hower wrote:
  

Reply from Dan Gibson:


Tourmaline is a functional transactional memory simulator, which is  
intended to have extensible behavior for future expansion. Its  
default behavior makes no attempt to model a realistic timing nor  
interleaving of transactions -- it simply provides the bare minimum  
implentation of atomicity in the most trivial, simulator-magic way  
possible -- by literally disabling all (other) processors.

To my knowledge, the released version of tourmaline has never been  
used to collect viable research data -- it is a tool intended to  
enable warm-up of transactional applications, as well as to  
facilitate debugging of transactional applications seperate from the  
debugging of the timing simulator.

If you are interested in looking at running concurrent threads, you  
should look into implementing sub-classes of TransactionController,  
which Tourmaline uses to guarantee atomicity of transactions. There  
is a how-to guide in the README for tourmaline, called the  
'Transaction Controller Cookbook'.

Please note that the /timing/ runs for the LogTM family of work all  
used Ruby, not Tourmaline. However, tourmaline is a viable tool for  
(much) longer simulations at the cost of some timing fidelity.

Regards,
Dan


On Sep 28, 2007, at 3:37 AM, horsnelm@xxxxxxxxxxxx wrote:

    

Hi,

I've been looking at the code inside the tourmaline TM module of  
the gems
package. I'm trying to run some benchmarks, in particular the  
stamp-0.9.4
benchmarks, and wondered if you could comment on the scheduling  
policy and
how you have used tourmaline, or gems in general, to generate your
results.

I can see in the tourmaline code, that when you begin a transaction  
you
disable interrupts in the processor registers, which means that  
until the
transaction resolves it cannot be interrupted. You switch back on
interrupts when the transaction commits or aborts.

Is it not the case that the operating system threads will  
interleave with
the transactions, competing for the cpu time? Do you prevent this from
happening by changing the scheduling policy in the OS, or do you  
measure
your results in some other manner? The reason I ask, is that when  
running
say a 4 threaded application, on a 4 cpu architecture, transactions
infrequently overlap as they are scheduled according to the OS.  
Ideally
I'd like the transactional threads to run as concurrently as  
possible to
look at the interactions between them.

Thanks,

Matt Horsnell
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