Hello everyone, We have taken a look at how pd-gem5 compares with multi-gem5. While intending to deliver the same functionality, there are some crucial differences:
* Synchronization. pd-gem5 implements this in Python (not a problem in itself; aesthetically this is nice, but...). The issue is that pd-gem5's data packets and barrier messages travel over different sockets. Since pd-gem5 could see data packets passing synchronization barriers, it could create an inconsistent checkpoint. multi-gem5's synchronization is implemented in C++ using sync events, but more importantly, the messages queue up in the same stream and so cannot have the issue just described. (Event ordering is often crucial in snapshot protocols.) Therefore we feel that multi-gem5 is a more robust solution in this respect. * Packet handling. pd-gem5 uses EtherTap for data packets but changed the polling mechanism to go through the main event queue. Since this rate is actually linked with simulator progress, it cannot guarantee that the packets are serviced at regular intervals of real time. This can lead to packets queueing up which would contribute to the synchronization issues mentioned above. multi-gem5 uses plain sockets with separate receive threads and so does not have this issue. * Checkpoint accuracy. A user would like to have a checkpoint at precisely the time the 'm5 checkpoint' operation is executed so as to not miss any of the area of interest in his application. pd-gem5 requires that simulation finish the current quantum before checkpointing, so it cannot provide this. (Shortening the quantum can help, but usually the snapshot is being taken while 'fast-forwarding', i.e. simulating as fast as possible, which would motivate a longer quantum.) multi-gem5 can enter the drain cycle immediately upon receiving a checkpoint request. We find this accuracy highly desirable. * Implementation of network topology. pd-gem5 uses a separate gem5 process to act as a switch whereas multi-gem5 uses a standalone packet relay process. We haven't measured the overhead of pd-gem5's simulated switch yet, but we're confident that our approach is at least as fast and more scalable. Thanks, Curtis ________________________________________ From: gem5-dev [[email protected]] On Behalf Of Mohammad Alian [[email protected]] Sent: Friday, June 26, 2015 7:37 PM To: gem5 Developer List Subject: Re: [gem5-dev] pd-gem5: simulating a parallel/distributed system on multiple physical hosts Hi Anthony, I think that would be a good option, then I can add pd-gem5 functionality on top of that. Right now I've simplified your implementation. Also, I think I had found some bugs in your patch that I cannot remember now. If you decided to ship EtherSwitch patch, let me know to give you a review on that. Thanks, Mohammad On Thu, Jun 25, 2015 at 8:36 PM, Gutierrez, Anthony < [email protected]> wrote: > Would it make sense for me to ship the EtherSwitch patch first, since it > has utility on its own, and then we can decide which of the "multi-gem5" > approaches is best, or if it's some combination of both? > > The only reason I never shipped it was because Steve raised an issue that > I didn't have a good alternative for, and didn't have the time to look into > one at that time. > ________________________________________ > From: gem5-dev [[email protected]] on behalf of Mohammad Alian [ > [email protected]] > Sent: Wednesday, June 24, 2015 12:43 PM > To: gem5 Developer List > Subject: Re: [gem5-dev] pd-gem5: simulating a parallel/distributed system > on multiple physical hosts > > Hi Andreas, > > Thanks for the comment. > I think the checkpointing support in both works is the same. Here is how > checkpointing support is implemented in pd-gem5: > > Whenever one of gem5 processes encounter an m5-checkpoint pseudo > instruction, it will send a “recv-ckpt” signal to the > “barrier” process. Then the “barrier” process sends a “take-ckpt” signal to > all the simulated nodes > (including the node that encountered m5-checkpoint) at the end of the > current simulation quantum. On the reception of > “take-ckpt” signal, gem5 processes start dumping check-points. This makes > each simulated node dump a checkpoint > at the same simulated time point while ensuring there is no in-flight > packets. > > I believe this is the same as multi-gem5 patch approach for checkpoint > support (based on the commit message of http://reviews.gem5.org/r/2865/). > Also, we have tested our mechanism with several benchmarks and it works. As > Steve suggested, I'll look into Curtis's patch and try to review it as > well. > But as Nilay also mentioned earlier, there are some codes missing in > Curtis's patch. I prefer to first run multi-gem5 before starting to review > it. > > Thank you, > Mohammad > > On Wed, Jun 24, 2015 at 7:25 AM, Andreas Hansson <[email protected]> > wrote: > > > Hi Steve, > > > > Apologies for the confusion. We are on the same page. My point is that we > > cannot simply take a little bit of patch A and a little bit of patch B. > > This change involves a lot of code, and we need to approach this in a > > structured fashion. My proposal is to do it bottom up, and start by > > getting the basic support in place. Since > http://reviews.gem5.org/r/2826/ > > has already been on the review board for a few months, I am merely > > suggesting that the it would be a good start to relate the newly posted > > patches to what is already there. > > > > Andreas > > > > > > > > On 24/06/2015 13:11, "gem5-dev on behalf of Steve Reinhardt" > > <[email protected] on behalf of [email protected]> wrote: > > > > >Hi Andreas, > > > > > >I'm a little confused by your email---you say you're fundamentally > opposed > > >to looking at both patches and picking the best features, then you point > > >out that the patches Curtis posted have the feature of better > > >checkpointing > > >support so we should pick that :). > > > > > >Obviously we can't just pick patch A from Mohammad's set and patch B > from > > >Curtis's set and expect them to work together, but I think that having > > >both > > >sets of patches available and comparing and contrasting the two > > >implementations should enable us to get to a single implementation > that's > > >the best of both. Someone will have to make the effort of integrating > the > > >better ideas from one set into the other set to create a new unified set > > >of > > >patches; (or maybe we commit one set and then integrate the best of the > > >other set as patches on top of that), but the first step is to identify > > >what "the best of both" is. Having Mohammad look at Curtis's patches, > and > > >Curtis (or someone else from ARM) closely examine Mohammad's patches > would > > >be a great start. I intend to review them both, though unfortunately my > > >time has been scarce lately---I'm hoping to squeeze that in later this > > >week. > > > > > >Once we've had a few people look at both, we can discuss the pros and > cons > > >of each, then discuss the strategy for getting the best features in. So > > >far I've heard that Mohammad's patches have a better network model but > the > > >ARM patches have better checkpointing support; that seems like a good > > >start. > > > > > >Steve > > > > > >On Wed, Jun 24, 2015 at 12:26 AM Andreas Hansson < > [email protected] > > > > > >wrote: > > > > > >> Hi all, > > >> > > >> Great work. However, I fundamentally do not believe in the approach of > > >> ‘letting reviewers pick the best features’. There is no way we would > > >>ever > > >> get something working out if it. We need to get _one_ working solution > > >> here, and figure out how to best get there. I would propose to do it > > >> bottom up, starting with the basic multi-simulator instance support, > > >> checkpointing support, and then move on to the network between the > > >> simulator instances. > > >> > > >> Thus, I propose we go with the low-level plumbing and checkpoint > support > > >> from what Curtis has posted. I believe proper checkpointing support to > > >>be > > >> the most challenging, and from what I can tell this is far more > limited > > >>in > > >> what you just posted Mohammad. Could you perhaps review Curtis patches > > >> based on your insights, and we can try and get these patches in shape > > >>and > > >> committed asap. > > >> > > >> Once we have the baseline functionality in place, then we can start > > >> looking at the more elaborate network models. > > >> > > >> Does this sound reasonable? > > >> > > >> Thanks, > > >> > > >> Andreas > > >> > > >> On 24/06/2015 05:05, "gem5-dev on behalf of Mohammad Alian" > > >> <[email protected] on behalf of [email protected]> wrote: > > >> > > >> >Hello All, > > >> > > > >> >I have submitted a chain of patches which enables gem5 to simulate a > > >> >cluster on multiple physical hosts: > > >> > > > >> >http://reviews.gem5.org/r/2909/ > > >> >http://reviews.gem5.org/r/2910/ > > >> >http://reviews.gem5.org/r/2912/ > > >> >http://reviews.gem5.org/r/2913/ > > >> >http://reviews.gem5.org/r/2914/ <http://reviews.gem5.org/r/2914/> > > >> > > > >> >and a patch that contains run scripts for a simple experiment: > > >> >http://reviews.gem5.org/r/2915/ > > >> > > > >> >We have run several benchmarks using this infrastructure, including > NAS > > >> >parallel benchmarks (MPI) and DCBench-hadoop > > >> >(http://prof.ict.ac.cn/DCBench/), > > >> >and would be happy to share scripts/diskimages. > > >> > > > >> >We call this *pd-gem5*. *pd-gem5 *functionality is more or less the > > >>same > > >> >as > > >> >Curtis's patch for *multi-gem5.* However, I feel *pd-gem5 *network > > >>model > > >> >is > > >> >more thorough; it also enables modeling different network topologies. > > >> >Having both set of changes together let reviewers to pick best > features > > >> >from both works. > > >> > > > >> >Thank you, > > >> >Mohammad Alian > > >> >_______________________________________________ > > >> >gem5-dev mailing list > > >> >[email protected] > > >> >http://m5sim.org/mailman/listinfo/gem5-dev > > >> > > >> > > >> -- IMPORTANT NOTICE: The contents of this email and any attachments > are > > >> confidential and may also be privileged. If you are not the intended > > >> recipient, please notify the sender immediately and do not disclose > the > > >> contents to any other person, use it for any purpose, or store or copy > > >>the > > >> information in any medium. Thank you. > > >> > > >> ARM Limited, Registered office 110 Fulbourn Road, Cambridge CB1 9NJ, > > >> Registered in England & Wales, Company No: 2557590 > > >> ARM Holdings plc, Registered office 110 Fulbourn Road, Cambridge CB1 > > >>9NJ, > > >> Registered in England & Wales, Company No: 2548782 > > >> _______________________________________________ > > >> gem5-dev mailing list > > >> [email protected] > > >> http://m5sim.org/mailman/listinfo/gem5-dev > > >> > > >_______________________________________________ > > >gem5-dev mailing list > > >[email protected] > > >http://m5sim.org/mailman/listinfo/gem5-dev > > > > > > -- IMPORTANT NOTICE: The contents of this email and any attachments are > > confidential and may also be privileged. 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