《Systems Performance: Enterprise and the Cloud》读书笔记系列（四） —— 第二章（三）

1. The degree to which more work is requested of a resource than it can process is saturation. Saturation begins to occur at 100% utilization (capacity-based), as extra work cannot be processed and begins to queue. Any degree of saturation is a performance issue, as time is spent waiting (latency).

2. In the field of performance, profiling is typically performed by sampling the state of the system at timed intervals, and then studying the set of samples.

3. Caching is frequently used to improve performance. A cache stores results from a slower storage tier in a faster storage tier for reference.  An example is caching disk blocks in main memory (RAM).

a) Cache's hit ratio: The number of times the needed data was found in the cache (hits) versus the number of times it was not(misses):  hit ratio = hit / total accesses (hits + misses);

b) Cache miss rate: Misses per second.

c) Total runtime for workload:  runtime = (hit rate * hit latency)+(niss rate * miss latency) (This calculation uses the average hit and miss latencies and assumes the work is serialized).

4. Cache state:

a) Cold: A cold cache is empty, or populated with unwanted data. The hit ratio for a cold cache is zero (or near zero as it begins to warm up).

b) Hot: A hot cache is populated with commonly requested data and has a high hit ratio, for example, over 99%.

c) Warm: A warm cache is one that is populated with useful data but doesn't have a high enough hit ratio to be considered hot.

d) Warmth: Cache warmth describes how hot or cold a cache is. An activity that improves cache warmth is one that aims to improve the cache hit ratio.