python: Remove redundant drain when changing memory modes

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

Changeset 10869:996b20e55386
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python: Remove redundant drain when changing memory modes

When the Python helper code switches CPU models, it sometimes also
needs to change the memory mode of the simulator. When this happens,
it accidentally tried to drain the simulator despite having done so
already. This changeset removes the redundant drain.

Testing Done

Long regressions (including switcheroos pass)

Ship It!

So is the assumption here that, if you're switching memory modes, you will always call switchCpus with do_drain=True? Other than that I don't see where the redundant drain is coming from.

Also, if the system is already drained, how long does a redundant drain take? That is, how much are we really saving here?

Steve Reinhardt July 4, 2015, 10:36 a.m. (July 4, 2015, 10:36 a.m.)

OK, looking at patch 2874 I see that this code will be changing anyway. So I think this patch makes sense in the context of the future implementation. I'm still not sure it makes sense in the context of the current implementation, but that's moot. I'd feel slightly better if you reordered the patches to make this one come after 2874 though...

Andreas Sandberg July 5, 2015, 3:15 a.m. (July 5, 2015, 3:15 a.m.)

The assumption is, and has always been, that a system is drained when the CPU switch takes place. All CPU models, including the atomic CPU require this. The reason we have do_drain as a keyword parameter is because there were cases (particularly in test cases where we test CPU switching and checkpoints) when we wanted to explicitly call drain and perform multiple operations that required a drained simulator in one go.

In practice, the cost of an additional drain is probably not huge, but it's not needed since the system should be drained already so I'd argue that it should go.

You have a pending review.

Review Board 2.0.15

This change has been marked as submitted.

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Status: Closed (submitted)