Some of the various things that Linux supports (such as network drivers, file systems, network protocols, etc.) can be in a state of development where the functionality, stability, or the level of testing is not yet high enough for general use. This is usually known as the "alpha-test" phase among developers. If a feature is currently in alpha-test, then the developers usually discourage uninformed widespread use of this feature by the general public to avoid "Why doesn't this work?" type mail messages. However, active testing and use of these systems is welcomed. Just be aware that it may not meet the normal level of reliability or it may fail to work in some special cases. Detailed bug reports from people familiar with the kernel internals are usually welcomed by the developers (before submitting bug reports, please read the documents <file:README>, <file:MAINTAINERS>, <file:REPORTING-BUGS>, <file:Documentation/BUG-HUNTING>, and <file:Documentation/oops-tracing.txt> in the kernel source). This option will also make obsoleted drivers available. These are drivers that have been replaced by something else, and/or are scheduled to be removed in a future kernel release. Unless you intend to help test and develop a feature or driver that falls into this category, or you have a situation that requires using these features, you should probably say N here, which will cause the configurator to present you with fewer choices. If you say Y here, you will be offered the choice of using features or drivers that are currently considered to be in the alpha-test phase.
Maximum of each of the number of arguments and environment variables passed to init from the kernel command line.
Same as running 'make CROSS_COMPILE=prefix-' but stored for default make runs in this kernel build directory. You don't need to set this unless you want the configured kernel build directory to select the cross-compiler automatically.
Append an extra string to the end of your kernel version. This will show up when you type uname, for example. The string you set here will be appended after the contents of any files with a filename matching localversion* in your object and source tree, in that order. Your total string can be a maximum of 64 characters.
This will try to automatically determine if the current tree is a release tree by looking for git tags that belong to the current top of tree revision. A string of the format -gxxxxxxxx will be added to the localversion if a git-based tree is found. The string generated by this will be appended after any matching localversion* files, and after the value set in CONFIG_LOCALVERSION. (The actual string used here is the first eight characters produced by running the command: $ git rev-parse --verify HEAD which is done within the script "scripts/setlocalversion".)
The linux kernel is a kind of self-extracting executable. Several compression algorithms are available, which differ in efficiency, compression and decompression speed. Compression speed is only relevant when building a kernel. Decompression speed is relevant at each boot. If you have any problems with bzip2 or lzma compressed kernels, mail me (Alain Knaff) <alain@knaff.lu>. (An older version of this functionality (bzip2 only), for 2.4, was supplied by Christian Ludwig) High compression options are mostly useful for users, who are low on disk space (embedded systems), but for whom ram size matters less. If in doubt, select 'gzip'
The old and tried gzip compression. It provides a good balance between compression ratio and decompression speed.
Its compression ratio and speed is intermediate. Decompression speed is slowest among the three. The kernel size is about 10% smaller with bzip2, in comparison to gzip. Bzip2 uses a large amount of memory. For modern kernels you will need at least 8MB RAM or more for booting.
The most recent compression algorithm. Its ratio is best, decompression speed is between the other two. Compression is slowest. The kernel size is about 33% smaller with LZMA in comparison to gzip.
XZ uses the LZMA2 algorithm and instruction set specific BCJ filters which can improve compression ratio of executable code. The size of the kernel is about 30% smaller with XZ in comparison to gzip. On architectures for which there is a BCJ filter (i386, x86_64, ARM, IA-64, PowerPC, and SPARC), XZ will create a few percent smaller kernel than plain LZMA. The speed is about the same as with LZMA: The decompression speed of XZ is better than that of bzip2 but worse than gzip and LZO. Compression is slow.
Its compression ratio is the poorest among the 4. The kernel size is about 10% bigger than gzip; however its speed (both compression and decompression) is the fastest.
This option determines the default system hostname before userspace calls sethostname(2). The kernel traditionally uses "(none)" here, but you may wish to use a different default here to make a minimal system more usable with less configuration.
This option allows you to choose whether you want to have support for so called swap devices or swap files in your kernel that are used to provide more virtual memory than the actual RAM present in your computer. If unsure say Y.
Inter Process Communication is a suite of library functions and system calls which let processes (running programs) synchronize and exchange information. It is generally considered to be a good thing, and some programs won't run unless you say Y here. In particular, if you want to run the DOS emulator dosemu under Linux (read the DOSEMU-HOWTO, available from <http://www.tldp.org/docs.html#howto>), you'll need to say Y here. You can find documentation about IPC with "info ipc" and also in section 6.4 of the Linux Programmer's Guide, available from <http://www.tldp.org/guides.html>.
POSIX variant of message queues is a part of IPC. In POSIX message queues every message has a priority which decides about succession of receiving it by a process. If you want to compile and run programs written e.g. for Solaris with use of its POSIX message queues (functions mq_*) say Y here. POSIX message queues are visible as a filesystem called 'mqueue' and can be mounted somewhere if you want to do filesystem operations on message queues. If unsure, say Y.
If you say Y here, a user level program will be able to instruct the kernel (via a special system call) to write process accounting information to a file: whenever a process exits, information about that process will be appended to the file by the kernel. The information includes things such as creation time, owning user, command name, memory usage, controlling terminal etc. (the complete list is in the struct acct in <file:include/linux/acct.h>). It is up to the user level program to do useful things with this information. This is generally a good idea, so say Y.
If you say Y here, the process accounting information is written in a new file format that also logs the process IDs of each process and it's parent. Note that this file format is incompatible with previous v0/v1/v2 file formats, so you will need updated tools for processing it. A preliminary version of these tools is available at <http://www.gnu.org/software/acct/>.
If you say Y here, a user level program will be able to map file names to handle and then later use the handle for different file system operations. This is useful in implementing userspace file servers, which now track files using handles instead of names. The handle would remain the same even if file names get renamed. Enables open_by_handle_at(2) and name_to_handle_at(2) syscalls.
Export selected statistics for tasks/processes through the generic netlink interface. Unlike BSD process accounting, the statistics are available during the lifetime of tasks/processes as responses to commands. Like BSD accounting, they are sent to user space on task exit. Say N if unsure.
Collect information on time spent by a task waiting for system resources like cpu, synchronous block I/O completion and swapping in pages. Such statistics can help in setting a task's priorities relative to other tasks for cpu, io, rss limits etc. Say N if unsure.
Collect extended task accounting data and send the data to userland for processing over the taskstats interface. Say N if unsure.
Collect information on the number of bytes of storage I/O which this task has caused. Say N if unsure.
Enable auditing infrastructure that can be used with another kernel subsystem, such as SELinux (which requires this for logging of avc messages output). Does not do system-call auditing without CONFIG_AUDITSYSCALL.
Enable low-overhead system-call auditing infrastructure that can be used independently or with another kernel subsystem, such as SELinux.
This option selects the RCU implementation that is designed for very large SMP system with hundreds or thousands of CPUs. It also scales down nicely to smaller systems.
This option selects the RCU implementation that is designed for very large SMP systems with hundreds or thousands of CPUs, but for which real-time response is also required. It also scales down nicely to smaller systems.
This option selects the RCU implementation that is designed for UP systems from which real-time response is not required. This option greatly reduces the memory footprint of RCU.
This option selects the RCU implementation that is designed for real-time UP systems. This option greatly reduces the memory footprint of RCU.
This option enables preemptible-RCU code that is common between the TREE_PREEMPT_RCU and TINY_PREEMPT_RCU implementations.
This option provides tracing in RCU which presents stats in debugfs for debugging RCU implementation. Say Y here if you want to enable RCU tracing Say N if you are unsure.
This option controls the fanout of hierarchical implementations of RCU, allowing RCU to work efficiently on machines with large numbers of CPUs. This value must be at least the fourth root of NR_CPUS, which allows NR_CPUS to be insanely large. The default value of RCU_FANOUT should be used for production systems, but if you are stress-testing the RCU implementation itself, small RCU_FANOUT values allow you to test large-system code paths on small(er) systems. Select a specific number if testing RCU itself. Take the default if unsure.
This option forces use of the exact RCU_FANOUT value specified, regardless of imbalances in the hierarchy. This is useful for testing RCU itself, and might one day be useful on systems with strong NUMA behavior. Without RCU_FANOUT_EXACT, the code will balance the hierarchy. Say N if unsure.
This option causes RCU to attempt to accelerate grace periods in order to allow the final CPU to enter dynticks-idle state more quickly. On the other hand, this option increases the overhead of the dynticks-idle checking, particularly on systems with large numbers of CPUs. Say Y if energy efficiency is critically important, particularly if you have relatively few CPUs. Say N if you are unsure.
This option provides tracing for the TREE_RCU and TREE_PREEMPT_RCU implementations, permitting Makefile to trivially select kernel/rcutree_trace.c.
This option boosts the priority of preempted RCU readers that block the current preemptible RCU grace period for too long. This option also prevents heavy loads from blocking RCU callback invocation for all flavors of RCU. Say Y here if you are working with real-time apps or heavy loads Say N here if you are unsure.
This option specifies the real-time priority to which preempted RCU readers are to be boosted. If you are working with CPU-bound real-time applications, you should specify a priority higher then the highest-priority CPU-bound application. Specify the real-time priority, or take the default if unsure.
This option specifies the time to wait after the beginning of a given grace period before priority-boosting preempted RCU readers blocking that grace period. Note that any RCU reader blocking an expedited RCU grace period is boosted immediately. Accept the default if unsure.
This option enables the complete Linux kernel ".config" file contents to be saved in the kernel. It provides documentation of which kernel options are used in a running kernel or in an on-disk kernel. This information can be extracted from the kernel image file with the script scripts/extract-ikconfig and used as input to rebuild the current kernel or to build another kernel. It can also be extracted from a running kernel by reading /proc/config.gz if enabled (below).
This option enables access to the kernel configuration file through /proc/config.gz.
Select kernel log buffer size as a power of 2. Examples: 17 => 128 KB 16 => 64 KB 15 => 32 KB 14 => 16 KB 13 => 8 KB 12 => 4 KB
This option adds support for grouping sets of processes together, for use with process control subsystems such as Cpusets, CFS, memory controls or device isolation. See - Documentation/scheduler/sched-design-CFS.txt (CFS) - Documentation/cgroups/ (features for grouping, isolation and resource control) Say N if unsure.
This option enables a simple cgroup subsystem that exports useful debugging information about the cgroups framework. Say N if unsure.
Provides a way to freeze and unfreeze all tasks in a cgroup.
Provides a cgroup implementing whitelists for devices which a process in the cgroup can mknod or open.
This option will let you create and manage CPUSETs which allow dynamically partitioning a system into sets of CPUs and Memory Nodes and assigning tasks to run only within those sets. This is primarily useful on large SMP or NUMA systems. Say N if unsure.
Provides a simple Resource Controller for monitoring the total CPU consumed by the tasks in a cgroup.
This option enables controller independent resource accounting infrastructure that works with cgroups.
Provides a memory resource controller that manages both anonymous memory and page cache. (See Documentation/cgroups/memory.txt) Note that setting this option increases fixed memory overhead associated with each page of memory in the system. By this, 20(40)bytes/PAGE_SIZE on 32(64)bit system will be occupied by memory usage tracking struct at boot. Total amount of this is printed out at boot. Only enable when you're ok with these trade offs and really sure you need the memory resource controller. Even when you enable this, you can set "cgroup_disable=memory" at your boot option to disable memory resource controller and you can avoid overheads. (and lose benefits of memory resource controller) This config option also selects MM_OWNER config option, which could in turn add some fork/exit overhead.
Add swap management feature to memory resource controller. When you enable this, you can limit mem+swap usage per cgroup. In other words, when you disable this, memory resource controller has no cares to usage of swap...a process can exhaust all of the swap. This extension is useful when you want to avoid exhaustion swap but this itself adds more overheads and consumes memory for remembering information. Especially if you use 32bit system or small memory system, please be careful about enabling this. When memory resource controller is disabled by boot option, this will be automatically disabled and there will be no overhead from this. Even when you set this config=y, if boot option "noswapaccount" is set, swap will not be accounted. Now, memory usage of swap_cgroup is 2 bytes per entry. If swap page size is 4096bytes, 512k per 1Gbytes of swap.
Memory Resource Controller Swap Extension comes with its price in a bigger memory consumption. General purpose distribution kernels which want to enable the feature but keep it disabled by default and let the user enable it by swapaccount boot command line parameter should have this option unselected. For those who want to have the feature enabled by default should select this option (if, for some reason, they need to disable it then noswapaccount does the trick).
This option extends the per-cpu mode to restrict monitoring to threads which belong to the cgroup specified and run on the designated cpu. Say N if unsure.
This feature lets CPU scheduler recognize task groups and control CPU bandwidth allocation to such task groups. It uses cgroups to group tasks.
This feature lets you explicitly allocate real CPU bandwidth to task groups. If enabled, it will also make it impossible to schedule realtime tasks for non-root users until you allocate realtime bandwidth for them. See Documentation/scheduler/sched-rt-group.txt for more information.
Generic block IO controller cgroup interface. This is the common cgroup interface which should be used by various IO controlling policies. Currently, CFQ IO scheduler uses it to recognize task groups and control disk bandwidth allocation (proportional time slice allocation) to such task groups. It is also used by bio throttling logic in block layer to implement upper limit in IO rates on a device. This option only enables generic Block IO controller infrastructure. One needs to also enable actual IO controlling logic/policy. For enabling proportional weight division of disk bandwidth in CFQ, set CONFIG_CFQ_GROUP_IOSCHED=y; for enabling throttling policy, set CONFIG_BLK_DEV_THROTTLING=y. See Documentation/cgroups/blkio-controller.txt for more information.
Enable some debugging help. Currently it exports additional stat files in a cgroup which can be useful for debugging.
Provides the way to make tasks work with different objects using the same id. For example same IPC id may refer to different objects or same user id or pid may refer to different tasks when used in different namespaces.
In this namespace tasks see different info provided with the uname() system call
In this namespace tasks work with IPC ids which correspond to different IPC objects in different namespaces.
This allows containers, i.e. vservers, to use user namespaces to provide different user info for different servers. If unsure, say N.
Support process id namespaces. This allows having multiple processes with the same pid as long as they are in different pid namespaces. This is a building block of containers.
Allow user space to create what appear to be multiple instances of the network stack.
This option optimizes the scheduler for common desktop workloads by automatically creating and populating task groups. This separation of workloads isolates aggressive CPU burners (like build jobs) from desktop applications. Task group autogeneration is currently based upon task session.
This option adds code that switches the layout of the "block" class devices, to not show up in /sys/class/block/, but only in /sys/block/. This switch is only active when the sysfs.deprecated=1 boot option is passed or the SYSFS_DEPRECATED_V2 option is set. This option allows new kernels to run on old distributions and tools, which might get confused by /sys/class/block/. Since 2007/2008 all major distributions and tools handle this just fine. Recent distributions and userspace tools after 2009/2010 depend on the existence of /sys/class/block/, and will not work with this option enabled. Only if you are using a new kernel on an old distribution, you might need to say Y here.
Enable deprecated sysfs by default. See the CONFIG_SYSFS_DEPRECATED option for more details about this option. Only if you are using a new kernel on an old distribution, you might need to say Y here. Even then, odds are you would not need it enabled, you can always pass the boot option if absolutely necessary.
This option enables support for relay interface support in certain file systems (such as debugfs). It is designed to provide an efficient mechanism for tools and facilities to relay large amounts of data from kernel space to user space. If unsure, say N.
The initial RAM filesystem is a ramfs which is loaded by the boot loader (loadlin or lilo) and that is mounted as root before the normal boot procedure. It is typically used to load modules needed to mount the "real" root file system, etc. See <file:Documentation/initrd.txt> for details. If RAM disk support (BLK_DEV_RAM) is also included, this also enables initial RAM disk (initrd) support and adds 15 Kbytes (more on some other architectures) to the kernel size. If unsure say Y.
Enabling this option will pass "-Os" instead of "-O2" to gcc resulting in a smaller kernel. If unsure, say Y.
This option allows certain base kernel options and settings to be disabled or tweaked. This is for specialized environments which can tolerate a "non-standard" kernel. Only use this if you really know what you are doing.
This enables the legacy 16-bit UID syscall wrappers.
sys_sysctl uses binary paths that have been found challenging to properly maintain and use. The interface in /proc/sys using paths with ascii names is now the primary path to this information. Almost nothing using the binary sysctl interface so if you are trying to save some space it is probably safe to disable this, making your kernel marginally smaller. If unsure say Y here.
Say Y here to let the kernel print out symbolic crash information and symbolic stack backtraces. This increases the size of the kernel somewhat, as all symbols have to be loaded into the kernel image.
Normally kallsyms only contains the symbols of functions for nicer OOPS messages and backtraces (i.e., symbols from the text and inittext sections). This is sufficient for most cases. And only in very rare cases (e.g., when a debugger is used) all symbols are required (e.g., names of variables from the data sections, etc). This option makes sure that all symbols are loaded into the kernel image (i.e., symbols from all sections) in cost of increased kernel size (depending on the kernel configuration, it may be 300KiB or something like this). Say N unless you really need all symbols.
This option is provided for the case where no hotplug or uevent capabilities is wanted by the kernel. You should only consider disabling this option for embedded systems that do not use modules, a dynamic /dev tree, or dynamic device discovery. Just say Y.
This option enables normal printk support. Removing it eliminates most of the message strings from the kernel image and makes the kernel more or less silent. As this makes it very difficult to diagnose system problems, saying N here is strongly discouraged.
Disabling this option eliminates support for BUG and WARN, reducing the size of your kernel image and potentially quietly ignoring numerous fatal conditions. You should only consider disabling this option for embedded systems with no facilities for reporting errors. Just say Y.
Enable support for generating core dumps. Disabling saves about 4k.
This option allows to disable the internal PC-Speaker support, saving some memory.
Disabling this option reduces the size of miscellaneous core kernel data structures. This saves memory on small machines, but may reduce performance.
Disabling this option will cause the kernel to be built without support for "fast userspace mutexes". The resulting kernel may not run glibc-based applications correctly.
Disabling this option will cause the kernel to be built without support for epoll family of system calls.
Enable the signalfd() system call that allows to receive signals on a file descriptor. If unsure, say Y.
Enable the timerfd() system call that allows to receive timer events on a file descriptor. If unsure, say Y.
Enable the eventfd() system call that allows to receive both kernel notification (ie. KAIO) or userspace notifications. If unsure, say Y.
The shmem is an internal filesystem used to manage shared memory. It is backed by swap and manages resource limits. It is also exported to userspace as tmpfs if TMPFS is enabled. Disabling this option replaces shmem and tmpfs with the much simpler ramfs code, which may be appropriate on small systems without swap.
This option enables POSIX asynchronous I/O which may by used by some high performance threaded applications. Disabling this option saves about 7k.
This option should be enabled if compiling the kernel for an embedded system so certain expert options are available for configuration.
See tools/perf/design.txt for details.
See tools/perf/design.txt for details
Enable kernel support for various performance events provided by software and hardware. Software events are supported either built-in or via the use of generic tracepoints. Most modern CPUs support performance events via performance counter registers. These registers count the number of certain types of hw events: such as instructions executed, cachemisses suffered, or branches mis-predicted - without slowing down the kernel or applications. These registers can also trigger interrupts when a threshold number of events have passed - and can thus be used to profile the code that runs on that CPU. The Linux Performance Event subsystem provides an abstraction of these software and hardware event capabilities, available via a system call and used by the "perf" utility in tools/perf/. It provides per task and per CPU counters, and it provides event capabilities on top of those. Say Y if unsure.
This config has been obsoleted by the PERF_EVENTS config option - please see that one for details. It has no effect on the kernel whether you enable it or not, it is a compatibility placeholder. Say N if unsure.
Use vmalloc memory to back perf mmap() buffers. Mostly useful for debugging the vmalloc code on platforms that don't require it. Say N if unsure.
VM event counters are needed for event counts to be shown. This option allows the disabling of the VM event counters on EXPERT systems. /proc/vmstat will only show page counts if VM event counters are disabled.
This enables workarounds for various PCI chipset bugs/quirks. Disable this only if your target machine is unaffected by PCI quirks.
SLUB has extensive debug support features. Disabling these can result in significant savings in code size. This also disables SLUB sysfs support. /sys/slab will not exist and there will be no support for cache validation etc.
Randomizing heap placement makes heap exploits harder, but it also breaks ancient binaries (including anything libc5 based). This option changes the bootup default to heap randomization disabled, and can be overridden at runtime by setting /proc/sys/kernel/randomize_va_space to 2. On non-ancient distros (post-2000 ones) N is usually a safe choice.
This option allows to select a slab allocator.
The regular slab allocator that is established and known to work well in all environments. It organizes cache hot objects in per cpu and per node queues.
SLUB is a slab allocator that minimizes cache line usage instead of managing queues of cached objects (SLAB approach). Per cpu caching is realized using slabs of objects instead of queues of objects. SLUB can use memory efficiently and has enhanced diagnostics. SLUB is the default choice for a slab allocator.
SLOB replaces the stock allocator with a drastically simpler allocator. SLOB is generally more space efficient but does not perform as well on large systems.
Normally, and according to the Linux spec, anonymous memory obtained from mmap() has it's contents cleared before it is passed to userspace. Enabling this config option allows you to request that mmap() skip that if it is given an MAP_UNINITIALIZED flag, thus providing a huge performance boost. If this option is not enabled, then the flag will be ignored. This is taken advantage of by uClibc's malloc(), and also by ELF-FDPIC binfmt's brk and stack allocator. Because of the obvious security issues, this option should only be enabled on embedded devices where you control what is run in userspace. Since that isn't generally a problem on no-MMU systems, it is normally safe to say Y here. See Documentation/nommu-mmap.txt for more information.
Say Y here to enable the extended profiling support mechanisms used by profilers such as OProfile.
Kernel modules are small pieces of compiled code which can be inserted in the running kernel, rather than being permanently built into the kernel. You use the "modprobe" tool to add (and sometimes remove) them. If you say Y here, many parts of the kernel can be built as modules (by answering M instead of Y where indicated): this is most useful for infrequently used options which are not required for booting. For more information, see the man pages for modprobe, lsmod, modinfo, insmod and rmmod. If you say Y here, you will need to run "make modules_install" to put the modules under /lib/modules/ where modprobe can find them (you may need to be root to do this). If unsure, say Y.
Allow loading of modules without version information (ie. modprobe --force). Forced module loading sets the 'F' (forced) taint flag and is usually a really bad idea.
Without this option you will not be able to unload any modules (note that some modules may not be unloadable anyway), which makes your kernel smaller, faster and simpler. If unsure, say Y.
This option allows you to force a module to unload, even if the kernel believes it is unsafe: the kernel will remove the module without waiting for anyone to stop using it (using the -f option to rmmod). This is mainly for kernel developers and desperate users. If unsure, say N.
Usually, you have to use modules compiled with your kernel. Saying Y here makes it sometimes possible to use modules compiled for different kernels, by adding enough information to the modules to (hopefully) spot any changes which would make them incompatible with the kernel you are running. If unsure, say N.
Modules which contain a MODULE_VERSION get an extra "srcversion" field inserted into their modinfo section, which contains a sum of the source files which made it. This helps maintainers see exactly which source was used to build a module (since others sometimes change the module source without updating the version). With this option, such a "srcversion" field will be created for all modules. If unsure, say N.
Back when each arch used to define their own cpu_online_map and cpu_possible_map, some of them chose to initialize cpu_possible_map with all 1s, and others with all 0s. When they were centralised, it was better to provide this option than to break all the archs and have several arch maintainers pursuing me down dark alleys.
Need stop_machine() primitive.