Why is egrep [wW][oO][rR][dD] faster than grep -i word

I've gotten an initial explanation about this test case from Stefan Seyfried, who wrote the paper this example was taken from. The problem here is that the CPU scheduler parts of cgroups always aims to keep any available CPU busy; it doesn't ever enforce a hard limit if everything will fit at once.

In the case where two processes (high and low here) are running on >=2 cores, it's just going to keep high on one core and low on the other. Both will then run all the time, at close to 100% usage, because they can do so without hitting the situation where the scheduler doesn't give them enough time on the CPU. cpu.share scheduling only happens if there's a shortage.

In the second case, both processes are pinned to the same CPU. Then the CPU sharing logic has to do something useful with the relative cpu.shares numbers to balance them out, and it does that as hoped.

Hard limits on CPU usage aren't likely to appear until after the CFS Bandwidth Control patch hits. At that point it may be possible to get something more like what I was hoping for.

Standalone printf

Part of the "expense" in invoking a process is that several things have to happen that are resource intensive.

1. The executable has to be loaded from the disk, this incurs slowness since the HDD has be be accessed in order to load the binary blob from the disk which the executable is stored as.
2. The executable is typically built using dynamic libraries, so some secondary files to the executable will also have to be loaded, (i.e. more binary blob data being read from the HDD).
3. Operating system overhead. Each process that you invoke incurs overhead in the form of a process ID having to be created for it. Space in memory will also have be carved out to both house the binary data being loaded from the HDD in steps 1 & 2, as well as multiple structures having to be populated to store things such as the processes' environment (environment variables etc.)

excerpt of an strace of /usr/bin/printf

    $ strace /usr/bin/printf "%s\n" "hello world"
    *execve("/usr/bin/printf", ["/usr/bin/printf", "%s\\n", "hello world"], [/* 91 vars */]) = 0
    brk(0)                                  = 0xe91000
    mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd155a6b000
    access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
    open("/etc/ld.so.cache", O_RDONLY)      = 3
    fstat(3, {st_mode=S_IFREG|0644, st_size=242452, ...}) = 0
    mmap(NULL, 242452, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fd155a2f000
    close(3)                                = 0
    open("/lib64/libc.so.6", O_RDONLY)      = 3
    read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0p\357!\3474\0\0\0"..., 832) = 832
    fstat(3, {st_mode=S_IFREG|0755, st_size=1956608, ...}) = 0
    mmap(0x34e7200000, 3781816, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x34e7200000
    mprotect(0x34e7391000, 2097152, PROT_NONE) = 0
    mmap(0x34e7591000, 20480, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x191000) = 0x34e7591000
    mmap(0x34e7596000, 21688, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x34e7596000
    close(3)                                = 0
    mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd155a2e000
    mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd155a2c000
    arch_prctl(ARCH_SET_FS, 0x7fd155a2c720) = 0
    mprotect(0x34e7591000, 16384, PROT_READ) = 0
    mprotect(0x34e701e000, 4096, PROT_READ) = 0
    munmap(0x7fd155a2f000, 242452)          = 0
    brk(0)                                  = 0xe91000
    brk(0xeb2000)                           = 0xeb2000
    brk(0)                                  = 0xeb2000
    open("/usr/lib/locale/locale-archive", O_RDONLY) = 3
    fstat(3, {st_mode=S_IFREG|0644, st_size=99158752, ...}) = 0
    mmap(NULL, 99158752, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fd14fb9b000
    close(3)                                = 0
    fstat(1, {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
    mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd155a6a000
    write(1, "hello world\n", 12hello world
    )           = 12
    close(1)                                = 0
    munmap(0x7fd155a6a000, 4096)            = 0
    close(2)                                = 0
    exit_group(0)                           = ?*

Looking through the above you can get a sense of the additional resources that /usr/bin/printf is having to incur due to it being a standalone executable.

Builtin printf

With the built version of printf all the libraries that it depends on as well as its binary blob have already been loaded into memory when Bash was invoked. So none of that has to be incurred again.

Effectively when you call the builtin "commands" to Bash, you're really making what amounts to a function call, since everything has already been loaded.

An analogy

If you've ever worked with a programming language, such as Perl, it's equivalent to making calls to the function (system("mycmd")) or using the backticks (`mycmd`). When you do either of those things, you're forking a separate process with it's own overhead, vs. using the functions that are offered to you through Perl's core functions.

Anatomy of Linux Process Management

There's a pretty good article on IBM Developerworks that breaks down the various aspects of how Linux processes are created and destroyed along with the different C libraries involved in the process. The article is titled:Anatomy of Linux process management - Creation, management, scheduling, and destruction. It's also available as a PDF.