Some questions about the results of Champsim

[Charlie039]

I wonder what is the difference between a PREFETCH ACCESS, PREFETCH REQUESTED and PREFETCH ISSUED. Shouldn't they be the same value? And what does PREFETCH HIT and MISS mean? I'm so confused about these definitions, I would be so grateful if you could give me some explanation, thx :)

[krishnamtibrewala]

PREFETCH REQUESTED : It will be number of time prefetcher_cache_operate from prefetcher implementation request for a prefetch
PREFETCH ISSUED : These are the request(PREFETCH ACCESS) which are actually issued. If your prefetch queue is full prefetch access is not issued.

With respect to
PREFETCH ACCESS / HIT / MISS
These are request coming from upper level (for example in case of L2C upper level will be L1I & L1D) so all the request goes in ACCESS, our of ACCESS it break down into hit & MISS

For more info refer https://github.com/ChampSim/ChampSim/blob/master/src/cache.cc , search for "sim_stats.pf_"

[krishnamtibrewala]

Hi @Charlie039 ,
Ans for your question are inline.

So as for L1I prefetch access, does it mean it is accessed by the registers(CPUs)(which is the upper level of L1 cache)?

You should look at the core_inst.inc file use the following cmd to find out the file find .csconfig -name core_inst.inc.
There could be multiple upper level for CACHE cpu0_L1I{ ... } , I am speculating you have an Instr. prefetcher in your L1I

So the more prefetch hit, the greater the performance(IPC) improvement is?

Perfect prefetch is not about getting 100%hit(since this imply what you are prefetching is in your cache), its more on line its not there and you want to bring it into the cache.

If the prefetch miss goes down to zero, does it mean 100% hit rate and a perfect prefetcher?

One way to look at this is your prefetcher is not doing a good job since the aim of prefetcher is bring cache line which are not there before the demand.

[ngober]

Perfect prefetch is not about getting 100%hit(since this imply what you are prefetching is in your cache), its more on line its not there and you want to bring it into the cache.

This is crucial. The "prefetch hits" statistic is "hits with type prefetch". This includes both prefetches issued at the given level as well as prefetch misses from upper levels. If a prefetch hits the cache, it is actually a non-useful (*) prefetch, since the line was already in the cache. In fact, prefetch misses are better insofar as they are useful prefetches (tracked separately), because they convert demand misses to earlier prefetch misses.

(*) Note, I continue to maintain that prefetches can be considered useful in a certain sense if it influences replacement policy, but that is a whole paper waiting to be written.

[Charlie039]

Thanks for your reply! So a prefetch hit means something already in cache,a prefetch miss means it's not in cache while prefetching but could be in when certain demand comes. So for some extend, the prefetch miss is a better indicator. Here's another two questions: 1) what's the relationship between prefetch access, useful & useless? In my point of view, prefetch access = (useful + useless + late prefetch(not shown) + not used nor evicted(not sure if it's useful or not )). Is that right? 2) The different between prefetch requested and prefetch issued comes from MSHR.occupancy right?(eg. If MSHR is full, a prefetch requested would not transform to a prefetch issued?) Then what's the difference between a prefetch issued and a prefetch access? They are far apart in value, but shouldn't the issued all become the access later on? Or is there certain constrain between them, like a prefetch buffer or what? Thanks again for your patience! :)

[ngober]

The difference between prefetches requested and issued is whether they are rejected due to queue exhaustion. The prefetch_line() function returns a boolean to indicate when this happens. If the MSHR is full, a prefetch will remain queued until resources are available. This may cause pressure on the prefetch queue if the prefetcher is very aggressive.

You are correct that late prefetches are not shown. A long time ago, I tinkered with the idea of displaying "MERGED" alongside hits and misses, but it didn't turn into anything (I wanted to say what it was merged from and into, and that was difficult to display properly). For now, MSHR merges just happen silently.

[Charlie039]

Thanks for your help!!!