On Thu, 7 Aug 2014, Somnath Roy wrote: > Hi Sage, > > I went through the tiering agent code base and here is my understanding on > the agent behavior. Please let me know if that is correct. > > ? > > 1.?????? Agent will be always in idle state if target_max_bytes or > target_max_objects not set on the pool irrespective of other tiering params > set in the pool. dirty_micro and full_micro will not be calculated if those > two params are zero which is by default I guess. Yeah > 2.?????? Now, flush will be activated if dirty_micro is > flush_target. My > understanding is, once it is activated it will iterate through all the dirty > objects and flush all the dirty objects which is > cache_min_flush_age. Am I > right ? Yeah > 3.?????? For the eviction, if full_micro > 1000000 , the mode set as > TierAgentState::EVICT_MODE_FULL and all the clean objects are flushed. Yeah > 4.?????? if (full_micro > evict_target), the mode is set as > TierAgentState::EVICT_MODE_SOME. In this scenario evict_effort is calculated > and based on hit_set and temp calculation some clean objects are evicted. My > question is , can we quantify this value ? For ex, if the target_full_ratio > = 50%, once the eviction is triggered, what %objects will be evicted ? The effort is calculated based on how far between target_full and 100% we are. This is mapped onto the estimation of atime. We generate a histogram of age for the objects we have examined, so after the agent has run a bit we'll have a reasonable idea how the current object's age compares to others and can decide whether this is older or newer than the others; based on that we decide what to do. > 5.?????? Also, why we are multiplying with 1000000 always ? Is it because in > the histogram it is positioned as 0..1000000 ? Yeah, and we use "micro" units throughout to avoid doing any floating point. > 6.?????? I saw the cache_min_evict_age is not been used anywhere, am I > missing anything ? It's possible. The _min_ params are a bit dangerous because they can potentially confuse the cache agent (e.g., what if *all* objects are under the min? How/when do we decide to ignore the min, or, how/when do we give up trying to find an older object?). > 7.?????? The cache_min_flush_age will only be applicable if the flush is > triggered after crossing the dirty_threshold, right ? If dirty_threshold is > not breached, the flush age param is never checked. Right. > I need to understand the behavior so that we can test the Cache tiering > properly. > > Is there any behavior I am missing here which you want us to test out ? There is a new behavior just merged into master that controls our decision to promote data into the cache on read; see maybe_handle_cache(). That's not strictly agent behavior per se. Also, there is now a readforward mode that doesn't promote on read ever, based on our discussion about the performance of flash on read. Hope this helps! sage
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