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Date: Wed, 30 Oct 2013 07:51:38 -0400
From: Yaniv Sapir <yaniv@...pteva.com>
To: john-dev <john-dev@...ts.openwall.com>
Subject: Re: bcrypt-parallella on 64-core (was: Katja's weekly
 report #13)

The problem is not with certain registers, but with certain cores. Cores on
the 2nd row should not *load *or *fetch *from external memory. DMA reads
are OK.



On Wed, Oct 30, 2013 at 4:47 AM, Solar Designer <solar@...nwall.com> wrote:

> Hi Katja,
>
> On Tue, Oct 29, 2013 at 09:13:10PM +0100, Katja Malvoni wrote:
> > I run self test for bcrypt-parallella format 10 times and counted for
> each
> > core how many times it didn't start computation. Than I tried using only
> > cores that didn't start less than 3 times out of ten. Number of such
> cores
> > is 24.
> > Computation isn't started always but when it is, performance for those 24
> > cores is 1807 c/s (I'm transferring data buffer with data for all 64
> > cores).
> > After this I tried to use cores that failed 3 times (10 more so 34 in
> > total) but I tried 10 times and it didn't work - some cores always fail
> to
> > start computation.
>
> Thank you for trying this.  From Yaniv's replies, I was under impression
> that you could fully avoid the CPU bugs by not triggering the known
> problematic situations.  IIRC, some cores must not do reads from
> external memory to certain registers.  You could structure your code
> such that this never happens, although this might be tricky to do while
> staying with C (for most of your Epiphany side code).  gcc's ability to
> explicitly put certain local variables into specific registers might
> help.  The syntax is:
>
>         register suitabletype mylocalvar __asm__("registername");
>
> e.g.:
>
>         register void *p __asm__("r40");
>         register int i __asm__("r41");
>
> I am unsure whether this does or does not prevent gcc from possibly
> allocating these same registers to something else, though.
>
> Overall, making use of the buggy 64-core chip does start to feel too
> problematic, given that you need to spend time on the FPGA project at
> the same time.  So giving up on 64-core Epiphany until more reliable
> chips are available makes sense. ;-(
>
> Alexander
>



-- 
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Yaniv Sapir
Adapteva Inc.
1666 Massachusetts Ave, Suite 14
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Phone: (781)-328-0513 (x104)
Email: yaniv@...pteva.com
Web: www.adapteva.com
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