Hi Gautam, There are a number of limitations of SE mode including with respect to multithreading as I described above. You may be able to get around the functional access issue by extending the protocol to support functional accesses better (e.g., improving the implementation of `functionalRead` and `functionalWrite` in the cache controller). Otherwise, you can probably use the MI_example or MESI protocols as a good example of how to implement DMAs. Other than DMA, things *should* work in FS mode. As always, using the Ruby random tester is faster than waiting for linux boot to run into an error.
Cheers, Jason On Sat, Sep 3, 2022 at 1:10 PM Gautam Pathak <gautam.pat...@uwaterloo.ca> wrote: > Hi Prof. Jason, > > Thank you for the clarification. I further wanted to ask whether there is > any other way to do multithreaded application tests correctly in SE+ruby > mode? If not, then we'll be forced to use FS mode which afaik does not work > currently with MSI (our modified protocol will have to be changed again > because FS mode requires (?) DMA support). Do you have any advice for > things to watch out for while testing our multithreaded applications in > Full System mode? > > Thanks and Regards, > Gautam Pathak > ------------------------------ > *From:* Jason Lowe-Power <ja...@lowepower.com> > *Sent:* Friday, September 2, 2022 10:51 AM > *To:* The gem5 Users mailing list <gem5-users@gem5.org> > *Subject:* [gem5-users] Re: Trying to add barrier to threads example > > Hi Gautam, > > Functional accesses (i.e., fake/debug accesses that bypass all timing) and > Ruby do not play well together. Fundamentally, it is hard to know what is > the most up to date value or which value(s) you have to update when a cache > block is in an intermediate state. With MI_example and MESI, these > protocols probably have more support for functional reads than MSI, but > you're also just getting lucky with those. At some point, if you're doing > functional accesses, it's going to fail. > > In SE mode, to implement the fake system calls which execute *in the > simulator* instead of via the guest kernel executing on the simulated CPU, > we must use the fake/unrealistic/magic functional accesses. So, this is why > SE mode + multithreading + Ruby will almost always run into a problem at > some point. FS mode will not have this issue because it does not use > functional accesses (almost none at all) and nothing is "faked." The > simulated CPU executes all of the kernel system calls. > > Hopefully this answers your question. Let me know if I can provide more > info :) > > Cheers, > Jason > > On Thu, Sep 1, 2022 at 3:58 PM Gautam Pathak <gautam.pat...@uwaterloo.ca> > wrote: > > Hi All, > > I'm trying to add a custom thread barrier to the array_add function by > using __sync_fetch_and_add. The system I'm using is MSI using Ruby in SE > mode. When I run this, I get the following error: > build/X86/mem/ruby/system/RubyPort.cc:434: fatal: Ruby functional read > failed for address 0x15b918. > Till now I have observed that there is an open issue which mentions that > Ruby+multithreading+SE is not supported. The exact fault occurs in Futex > syscall. (Ref. https://gem5.atlassian.net/browse/GEM5-676). However, I > have tested the same for MI_Example protocol (using simple_ruby.py) and > MESI_Two_Level (using se.py) upto 30+ threads and it works. This makes me > wonder whether this is an MSI protocol bug. Any suggestions on how I should > go on about this issue would be appreciated! Otherwise, is Full System mode > the recommended option for Ruby+custom coherence protocol+multithreading > (possibly using atomics)? > > Thanks and Regards, > Gautam Pathak > _______________________________________________ > gem5-users mailing list -- gem5-users@gem5.org > To unsubscribe send an email to gem5-users-le...@gem5.org > > _______________________________________________ > gem5-users mailing list -- gem5-users@gem5.org > To unsubscribe send an email to gem5-users-le...@gem5.org >
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