AnandTech: No more mysteries: Apple's G5 versus x86, Mac OS X versus Linux

Mac OS X versus Linux

Lmbench 2.04 provides a suite of micro benchmarks that measure the bottlenecks at the Unix operating system and CPU level. This makes it very suitable for testing the theory that Mac OS X might be the culprit for the terrible server performance of the Apple platform.

Signals allow processes (and thus threads) to interrupt other processes. In a database system such as MySQL 4.x where so many processes/threads (60 in our MySQL screenshot) and many accesses to the kernel must be managed, signal handling is a critical performance factor.

Larry McVoy (SGI) and Carl Staelin (HP):

" Lmbench measure both signal installation and signal dispatching in two separate loops, within the context of one process. It measures signal handling by installing a signal handler and then repeatedly sending itself the signal."

Host	OS	Mhz	null	null call	open I/O	stat	slct clos	sig TCP	sig inst
Xeon 3.06 GHz	Linux 2.4	3056	0.42	0.63	4.47	5.58	18.2	0.68	2.33
G5 2.7 GHz	Darwin 8.1	2700	1.13	1.91	4.64	8.60	21.9	1.67	6.20
Xeon 3.6 GHz	Linux 2.6	3585	0.19	0.25	2.30	2.88	9.00	0.28	2.70
Opteron 850	Linux 2.6	2404	0.08	0.17	2.11	2.69	12.4	0.17	1.14

All numbers are expressed in microseconds, lower is thus better. First of all, you can see that kernel 2.6 is in most cases a lot more efficient. Secondly, although this is not the most accurate benchmark, the message is clear: the foundation of Mac OS X server, Darwin handles the signals the slowest. In some cases, Darwin is even several times slower.

As we increase the level of concurrency in our database test, many threads must be created. The Unix process/thread creation is called "forking" as a copy of the calling process is made.

lmbench "fork" measures simple process creation by creating a process and immediately exiting the child process. The parent process waits for the child process to exit. The benchmark is intended to measure the overhead for creating a new thread of control, so it includes the fork and the exit time.

lmbench "exec" measures the time to create a completely new process, while " sh" measures to start a new process and run a little program via /bin/ sh (complicated new process creation).

Host	OS	Mhz	fork hndl	exec proc	Sh proc
Xeon 3.06 GHz	Linux	3056	163	544	3021
G5 2.7 GHz	Darwin	2700	659	2308	4960
Xeon 3.6 GHz	Linux	3585	158	467	2688
Opteron 850	Linux	2404	125	471	2393

Mac OS X is incredibly slow, between 2 and 5(!) times slower, in creating new threads, as it doesn't use kernel threads, and has to go through extra layers (wrappers). No need to continue our search: the G5 might not be the fastest integer CPU on earth - its database performance is completely crippled by an asthmatic operating system that needs up to 5 times more time to handle and create threads.

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