Multithreaded file server workload

The effect of Hyper-Threading on the file server was measured with dbench and its companion test, tbench. dbench is similar to the well known NetBench benchmark from the Ziff-Davis Media benchmark program, which lets you measure the performance of file servers as they handle network file requests from clients. However, while NetBench requires an elaborate setup of actual physical clients, dbench simulates the 90,000 operations typically run by a NetBench client by sniffing a 4 MB file called client.txt to produce the same workload. The contents of this file are file operation directives such as SMBopenx, SMBclose, SMBwritebraw, SMBgetatr, etc. Those I/O calls correspond to the Server Message Protocol Block (SMB) that the SMBD server in SAMBA would produce in a netbench run. The SMB protocol is used by Microsoft Windows 3.11, NT and 95/98 to share disks and printers.

In our tests, a total of 18 different types of I/O calls were used including open file, read, write, lock, unlock, get file attribute, set file attribute, close, get disk free space, get file time, set file time, find open, find next, find close, rename file, delete file, create new file, and flush file buffer.

dbench can simulate any number of clients without going through the expense of a physical setup. dbench produces only the filesystem load, and it does no networking calls. During a run, each client records the number of bytes of data moved and divides this number by the amount of time required to move the data. All client throughput scores are then added up to determine the overall throughput for the server. The overall I/O throughput score represents the number of megabytes per second transferred during the test. This is a measurement of how well the server can handle file requests from clients.

dbench is a good test for Hyper-Threading because it creates a high load and activity on the CPU and I/O schedulers. The ability of Hyper-Threading to support multithreaded file serving is severely tested by dbench because many files are created and accessed simultaneously by the clients. Each client has to create about 21 megabytes worth of test data files. For a test run with 20 clients, about 420 megabytes of data are expected. dbench is considered a good test to measure the performance of the elevator algorithm used in the Linux filesystem. dbench is used to test the working correctness of the algorithm, and whether the elevator is aggressive enough. It is also an interesting test for page replacement.

Table 5 shows the impact of HT on the dbench workload. Each data point represents the geometric mean of five runs. The data indicates that Hyper-Threading would improve dbench from as little as 9% to as much as 29%. The overall improvement is 18% based on the geometric mean of the five test scenarios.

Table 5. Effects of Hyper-Threading on dbench throughput

Figure 2. Effects of Hyper-Threading on the dbench workload