Wednesday, August 12, 2009

Replaying traces, Part 4

So far we’ve replayed the same workload on 3 virtual servers, each with 4 CPUs, and 3 physical servers, 2 with 4 CPUs and 1 with 16 CPUs. We also ran one with 8 CPUs but that replay appeared to hang at 99%.

All the physical replays showed the same trends. A 16 CPU server performed better than 4 CPUs, Hyper-threading off performed better than hyper-threading on. The average for IO and transactions/sec followed the same curve; the longer the replay time the lower the average.

But the virtual server replays showed a different trend. All of these completed in less time

than the 4 CPU tests on a physical server. And the averages for IO and transactions/sec were noticeably less. Since the workload was the same in each case we expected the averages to be consistent with the trend set by the physical since the total number of transactions and IO should have bee the same across the board.

I’m guessing that there were more deadlocks on the virtual than the physical. In all cases the CPU usage was the same, averaging higher than 80%. If deadlock victims are rolled back it would explain the decrease in total IO. But I don’t know if a rolled back transaction is still considered a transaction by the Windows counters. This is going to take some research.

We have some more tests scheduled using a 64 bit virtual server and one test using VMWare’s newest host that supports virtual servers running 8 CPUs. 

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