Discussion:
How to modify the Cache Timing Model ?
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梁政
1970-01-01 00:00:00 UTC
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Hi




I am reading the latest code of gem5 and try to make the cache model more flexible (e.g., allowing non-constant access latency). So I will change the timing behavior of the Cache class.




Currently, I am reading the code in /mem/cache. I found that two major classes have their timing model: the Cache/NoncoherentCache/BaseCache family and the Tag family.




So what I need to do is to change related codes with a device timing model, right? Or there may be other points I missed? Thanks for your advice.




BTW, what are the FALRU tags for? It seems all configurations use SetAssoc Tags. I found a paper from UC.Berkeley. It is related to Sector Cache. Maybe someone will use that model in the future. But why should Fully-associative LRU cache be considered separately?





Regards

Zheng Liang

EECS, Peking University
Daniel Carvalho
2018-11-10 14:51:56 UTC
Permalink
Hello Liang,
The cache timing model is something that me, Jason and Nikos have been recently discussing. You can follow part of the discussion on the following links: https://gem5-review.googlesource.com/c/public/gem5/+/13697 and https://gem5-review.googlesource.com/c/public/gem5/+/13835.
Reworking the timing model to be more accurate and flexible is a hard task, and you will likely just want to modify code related to timing, as the latency in atomic mode is not well defined (in the sense of how correct it should be; There is no clear answer). I'd suggest you to look at these patches and their evolution to have and idea of the decisions that you may or may not want to take (I am not saying they are the right way to do it, though).

Regarding the Tags, we currently have 3 sub-classes: BaseSetAssoc, FALRU and SectorTags. Although we can think of FALRU as a subset of BaseSetAssoc (and you can definitely create a full associative tag using BaseSetAssoc), FALRU has its own implementation, which leverages from hashes and the awareness of a single replacement policy (LRU), because otherwise the cost of checking every block would be too high for greater cache sizes.

Regards,Daniel
Em sexta-feira, 9 de novembro de 2018 22:00:59 GMT+1, 梁政 <***@pku.edu.cn> escreveu:


Hi




I am reading the latest code of gem5 and try to make the cache model more flexible (e.g., allowing non-constant access latency). So I will change the timing behavior of the Cache class.




Currently, I am reading the code in /mem/cache. I found that two major classes have their timing model: the Cache/NoncoherentCache/BaseCache family and the Tag family.




So what I need to do is to change related codes with a device timing model, right? Or there may be other points I missed? Thanks for your advice.




BTW, what are the FALRU tags for? It seems all configurations use SetAssoc Tags. I found a paper from UC.Berkeley. It is related to Sector Cache. Maybe someone will use that model in the future. But why should Fully-associative LRU cache be considered separately?  





Regards

Zheng Liang

EECS, Peking University
梁政
1970-01-01 00:00:00 UTC
Permalink
Thanks a lot.




By the way, I want to know how to run regression tests. It seems the regression test depends on SPEC CPU2000 heavily. I have the SPEC CPU2000 & 2006 suite. But I still don't understand how to make it work...




The document site http://gem5.org/SPEC_benchmarks provides such information:




Workloads created from this package expect to find their binaries and input files in the following two directories, respectively.

cpu2000/binaries/<isa>/<operating system>/<benchmark>
cpu2000/data/<benchmark>/<input set>




But there isn't an example. To my understanding, an example of the mcf benchmark should be :

cpu2000/binaries/x86/linux/mcf

cpu2000/binaries/mcf/ref

cpu2000/binaries/mcf/ test


cpu2000/binaries/mcf/train

These are so-called directories. And there should be files in these directories. For instance, cpu2000/binaries/x86/linux/mcf/mcf, the latter mcf is a file.

Or maybe I should use cpu2000/binaries/x86/linux/mcf, just the mcf program, no additional path.

I have tried both methods but failed.

The regress.py just throws:

AttributeError: /home/zheng/SPEC2000_v13/binaries/x86/linux/mcf not found.

But the file is just there. I don't know what happened.




BTW, I have a question about the naming methods of gem5: when to use UPPERCASE in paths and file names? Sometimes, I need to use "X86", "ALPHA". While other times I need to use "x86", "alpha". Frankly speaking, it is terrible!




Best Regards

Zheng Liang

EECS, Peking University


-----Original Messages-----
From:"Daniel Carvalho" <***@yahoo.com.br>
Sent Time:2018-11-10 22:51:56 (Saturday)
To: gem5-***@gem5.org, "梁政" <***@pku.edu.cn>
Cc:
Subject: Re: How to modify the Cache Timing Model ?


Hello Liang,


The cache timing model is something that me, Jason and Nikos have been recently discussing. You can follow part of the discussion on the following links: https://gem5-review.googlesource.com/c/public/gem5/+/13697 and https://gem5-review.googlesource.com/c/public/gem5/+/13835.


Reworking the timing model to be more accurate and flexible is a hard task, and you will likely just want to modify code related to timing, as the latency in atomic mode is not well defined (in the sense of how correct it should be; There is no clear answer). I'd suggest you to look at these patches and their evolution to have and idea of the decisions that you may or may not want to take (I am not saying they are the right way to do it, though).



Regarding the Tags, we currently have 3 sub-classes: BaseSetAssoc, FALRU and SectorTags. Although we can think of FALRU as a subset of BaseSetAssoc (and you can definitely create a full associative tag using BaseSetAssoc), FALRU has its own implementation, which leverages from hashes and the awareness of a single replacement policy (LRU), because otherwise the cost of checking every block would be too high for greater cache sizes.



Regards,
Daniel

Em sexta-feira, 9 de novembro de 2018 22:00:59 GMT+1, 梁政 <***@pku.edu.cn> escreveu:





Hi




I am reading the latest code of gem5 and try to make the cache model more flexible (e.g., allowing non-constant access latency). So I will change the timing behavior of the Cache class.




Currently, I am reading the code in /mem/cache. I found that two major classes have their timing model: the Cache/NoncoherentCache/BaseCache family and the Tag family.




So what I need to do is to change related codes with a device timing model, right? Or there may be other points I missed? Thanks for your advice.




BTW, what are the FALRU tags for? It seems all configurations use SetAssoc Tags. I found a paper from UC.Berkeley. It is related to Sector Cache. Maybe someone will use that model in the future. But why should Fully-associative LRU cache be considered separately?





Regards

Zheng Liang

EECS, Peking University

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