Comparing performance with sysbench: memory, threads and mutexes


In the previous post, I gave some feedback on the cpu and fileio workload tests that sysbench can handle. Next on the agenda are the memory, threads and mutex workloads.

When using the memory workload, sysbench will allocate a buffer (provided through the --memory-block-size parameter, defaults to 1kbyte) and each execution will read or write to this memory (--memory-oper, defaults to write) in a random or sequential manner (--memory-access-mode, defaults to sequential).

$ sysbench --test=memory --memory-block-size=1M --memory-total-size=10G run
Host throughput, 1M:  3959,78 MB/sec
Guest throughput, 1M: 3079,29 MB/sec

The guest has a lower throughput (about 77% of the host), which is lower than what most online posts provide on KVM performance. We'll get back to that later. Let's look at the default block size of 1k (meaning that the benchmark will do a lot more executions before it reaches the total memory (in load):

$ sysbench --test=memory --memory-total-size=1G run
Host throughput, 1k:  1702,59 MB/sec
Guest throughput, 1k:   23,67 MB/sec

This is a lot worse: the guest' throughput is only 1,4% of the host throughput! The qemu-kvm process on the host is also taking up a lot of CPU.

Now let's take a look at the other workload, threads. In this particular workload, you identify the number of threads (--num-threads), the number of locks (--thread-locks) and the number of times a thread should run its 'lock-yield..unlock' workload (--thread-yields). The more locks you identify, the less number of threads will have the same lock (each thread is allocated a single lock during an execution, but every new execution will give it a new lock so the threads do not always take the same lock).

Note that parts of this is also handled by the other tests: mutex'es are used when a new operation (execution) for the thread is prepared. In case of the memory-related workload above, the smaller the buffer size, the more frequent thread operations are needed. In the last run we did (with the bad performance), millions of operations were executed (although no yields were performed). Something similar can be simulated using a single lock, single thread and a very high number of operations and no yields:

$ sysbench --test=threads --num-threads=1 --thread-yields=0 --max-requests=1000000 --thread-locks=1 run
Host runtime:    0,3267 s  (event:    0,2278)
Guest runtime:  40,7672 s  (event:   30,6084)

This means that the guest "throughput" problems from the memory identified above seem to be related to this rather than memory-specific regressions. To verify if the scheduler itself also shows regressions, we can run more threads concurrently. For instance, running 128 threads simultaneously, using the otherwise default settings, during 10 seconds:

$ sysbench --test=threads --num-threads=128 --max-time=10s run
Host:   9765 executions (events)
Guest:   512 executions (events)

Here we get only 5% throughput.

Let's focus on the mutex again, as sysbench has an additional mutex workload test. The workload has each thread running a local fast loop (simple increments, --mutex-loops) after which it takes a random mutex (one of --mutex-num), locks it, increments a global variable and then releases the mutex again. This is repeated for the number of locks identified (--mutex-locks). If mutex operations would be the cause of the performance issues above, then we would notice that the mutex operations are a major performance regression on my system.

Let's run that workload with a single thread (default), no loops and a single mutex.

$ sysbench --test=mutex --mutex-num=1 --mutex-locks=50000000 --mutex-loops=1 run
Host (duration):   2600,57 ms
Guest (duration):  2571,44 ms

In this example, we see that the mutex operations are almost at the same speed (99%) of the host, so pure mutex operations are not likely to be the cause of the performance regressions earlier on. So what does give the performance problems? Well, that investigation will be for the third and last post in this series ;-)