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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