Hi,
I am using the up-to-date gem5 version (repo.gem5.org/gem5). and run
the ALPHA arch with FS mode.
The system have many stalls due to lack of free ROB, IQ or LSQ,
although the simulation still procedes slowly.
I increase the ROB and IQ to 4048 entries each, but the stalls didn't
disappear. I increase the Load Queue to 4048 (I know it may be not
reasonable), but I cannot increase Store Queue to more than 128
entries, the simulation would halt.
I tried to debug with gem5.debug.
The simulation is aborted for "void BaseDynInst<
<template-parameter-1-1> >::initVars() [with Impl = O3CPUImpl]:
Assertion `cpu->instcount <= 1500' failed."
This was ignored when using gem5.fast earlier.
I see the other posts that it means the in flight instructions
increases beyond a reasonable limit
I have some questions I cannot understand.
1. Is it reasonable that the in flight instructions increases while I
use large ROB and IQ size? I was expecting in-flight instructions
didn't increase and the stalled caused by lack of buffer would
disappear.
2. I tried to debug it myself, and find that some instructions are not
destroyed after committing. It seems they are never destroyed, and
kept somewhere in some queue (I don't know which queue).
For most (regular) instructions, when
"instList.erase(removeList.front())" is called in o3/cpu.cc, the
instruction would be destroyed.
For some instructions, they are not destroyed properly after
calling instList.erase. I debug to find the reason is the reference
counter of the instruction object does not reach zero, and the system
would destroy the instruction object while nobody is still referencing
(using) this instruction. According to my understanding, there must be
somewhere in the pipeline that didn't decrease the reference
correctly, or some special operations (like squash) didn't call
functions to remove the reference.
3. I am not sure how this would cause the ROB and IQ to overflow (no
matter how big I set the values of these buffers).
Is there anyone see this kind of issues before? Is it really a bug or
did I do something wrong? If it is a bug, can you give me some hint to
detect which part may not have decrement the reference counter of the
instruction object? Any suspect?
Thanks,
Chao
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