Bus: Make the default bus width 8 bytes instead of 64

Information
Submitter:	Andreas Hansson
Repository:	gem5
Branch:	default
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Description

Changeset 9075:04e848b767d7
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Bus: Make the default bus width 8 bytes instead of 64

This patch changes the default bus width to a more sensible 8 bytes
(64 bits), which is in line with most on-chip buses. Although there
are cases where a wider or narrower bus is useful, the 8 bytes is a
good compromise to serve as the default.

This patch changes essentially all statistics, and will be bundled
with the outstanding changes to the bus.

Testing Done

util/regress all running and producing the right output (disregarding t1000 and eio) but with essentially all timing tests exhibiting stat differences

Ship It!

Ship It!

Steve Reinhardt July 4, 2012, 9:39 a.m. (July 4, 2012, 9:39 a.m.)

Actually I'm now having second thoughts about this.  The bottom line is that people really need to carefully specify bus bandwidths for a realistic system, and no one default is going to be realistic in all situations.  I agree that the current value is unrealistically high (not sure how we ended up with that), but I'm concerned that cutting it 8X to a mere 8 GBps is going to introduce a significant bottleneck on multicore systems, to the point where existing users will get quite surprised when things change dramatically on them.

One solution would be to get rid of the default width altogether (and maybe the default clock too) to force everyone to think about what they're doing.  If we couple that with making sure that reasonable widths & frequencies are specified in the config files, then it won't break any of our existing configs or tests.  You could even leave the width at 64 in the regression configs and avoid any impact there (though as you point out, you have to rerun regressions anyway so that's not a huge practical savings).

Andreas Hansson July 4, 2012, 5:44 p.m. (July 4, 2012, 5:44 p.m.)

I agree that it will change things quite dramatically, but on the other hand, the values that come out will be far more realistic with the 64-bit compared to the 512-bit bus width. Ultimately, if there is a bottleneck, then even the existing users should see it, right? I can send out a mail on the dev and user list highlighting the change to avoid (or at least lessen) surprises.

I think the 64-bit is a good option for a default, as it actually is the most common case in actual implementations (as far as I can tell at least). Even though removing the default might force users to think, the same could then be said for a lot of parameters in the system.

In conclusion, I'd vote for keeping the default, and changing it to 64 bits (8 bytes), and broadcast the change on the lists.

Andreas Hansson July 5, 2012, 5:22 p.m. (July 5, 2012, 5:22 p.m.)

What is the verdict? Anyone else having anything to add? Can I proceed and push this (and the ones before it) along with the stats updates?

Steve Reinhardt July 5, 2012, 11:10 p.m. (July 5, 2012, 11:10 p.m.)

Yes, I suppose since the performance will be changing due to the new bus architecture, now is as good a time as any to make the change.  An announcement on gem5-users is definitely in order.

You have a pending review.

Review Board 2.0.15

This change has been marked as submitted.

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Status: Closed (submitted)