可以看到設為 24GB (記憶體 75% 的量) 時很穩定的在 44K QPS 與 3.5ms (95%)：
This gives us about 44K QPS. The 95% query response time (reported by sysbench) is about 3.5ms.
而當設成 32GB 的時候開始可以觀察到 swap i/o，掉到 20K QPS 與 9ms (95%)：
We can see that performance stabilizes after a bit at around 20K QPS, with some 380MB/sec disk IO and 125MB/sec swap IO. The 95% query response time has grown to around 9ms.
當拉到 48GB 的時候就更掉更多，6K QPS 與 35ms (95%)：
Now we have around 6K QPS. Disk IO has dropped to 250MB/sec, and swap IO is up to 190MB/sec. The 95% query response time is around 35ms.
When I started, I expected severe performance drop even with very minor swapping. I surprised myself by getting swap activity to more than 100MB/sec, with performance “only” halved.
這邊測試用的是 SSD，如果是傳統用磁頭的硬碟，對 random access 應該會很敏感而掉更多：
This assumes your swap space is on an SSD, of course! SSDs handle random IO (which is what paging activity usually is) much better than HDDs.
基本上還是要避免碰到 swap 啦，另外 comment 的地方剛好有提到前陣子在猜測的 best practice，測試時的
1，這應該是作者的 best practice：
Swappiness was set to 1 in this case. I was not expecting this to cause significant impact as swapping is caused by genuine (intended) missconfiguration with more memory required than available.