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memory management – What to choose between Slab and Slub Allocator in Linux Kernel? – Stack Overflow

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• Summary of article content: Articles about memory management – What to choose between Slab and Slub Allocator in Linux Kernel? – Stack Overflow Slub is the next-generation replacement memory allocator, which has been the default in the Linux kernel since 2.6.23. It continues to employ … …
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SLUB (software) – Wikipedia

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• Summary of article content: Articles about SLUB (software) – Wikipedia SLUB (the unqueued slab allocator) is a memory management mechanism intended for the efficient memory allocation of kernel objects which displays the … …
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Short users guide for SLUB — The Linux Kernel documentation

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• Summary of article content: Articles about Short users guide for SLUB — The Linux Kernel documentation The basic philosophy of SLUB is very different from SLAB. SLAB requires rebuilding the kernel to activate debug options for all slab caches. …
• Most searched keywords: Whether you are looking for Short users guide for SLUB — The Linux Kernel documentation The basic philosophy of SLUB is very different from SLAB. SLAB requires rebuilding the kernel to activate debug options for all slab caches.
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How to use SLAB memory allocation instead of SLUB allocation in Linux kernel? – Unix & Linux Stack Exchange

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• Summary of article content: Articles about How to use SLAB memory allocation instead of SLUB allocation in Linux kernel? – Unix & Linux Stack Exchange There is no toggle in runtime: you switch between SLAB and SLUB (and if you enable EXPERT mode, SLOB) by recompiling the kernel. …
• Most searched keywords: Whether you are looking for How to use SLAB memory allocation instead of SLUB allocation in Linux kernel? – Unix & Linux Stack Exchange There is no toggle in runtime: you switch between SLAB and SLUB (and if you enable EXPERT mode, SLOB) by recompiling the kernel.
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The SLUB allocator [LWN.net]

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• Summary of article content: Articles about The SLUB allocator [LWN.net] In the SLUB allocator, a slab is simply a group of one or more pages neatly packed with objects of a given size. There is no metadata within … …
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Linux内核中的slab/slob/slub– 在搞晕前先记下来 – lucelu – 博客园

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• Summary of article content: Articles about Linux内核中的slab/slob/slub– 在搞晕前先记下来 – lucelu – 博客园 SLUB被很认为是Slab和Slob的取代者，大概在2.6.24/2.6.25将会被同志们抛弃。而SLUB将是未来Linux Kernel中的首选。 Linux内核真是变化太快了，内存管理这 … …
• Most searched keywords: Whether you are looking for Linux内核中的slab/slob/slub– 在搞晕前先记下来 – lucelu – 博客园 SLUB被很认为是Slab和Slob的取代者，大概在2.6.24/2.6.25将会被同志们抛弃。而SLUB将是未来Linux Kernel中的首选。 Linux内核真是变化太快了，内存管理这 … 很久很久以前：一个叫做Mark Hemment的哥儿们写了Slab。在接下来的一些年里，其他人对Slab进行了完善。一年半以前，SLOB问世了。SLOB的目标是针对嵌入式系统的，主要是适用于那些内存非
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The Slab Allocator in the Linux kernel

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The Slab Allocator in the Linux kernel

Introduction

The Buddy Allocator

The Slab Allocator

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What to choose between Slab and Slub Allocator in Linux Kernel?

In the search of answer, I posted the same question on Quora and Robert Love answered it:

I’m assuming you are asking this from the point-of-view of the user of a system, or perhaps someone building a kernel for a particular product. As a kernel developer, you don’t care what “slab” allocator is in use; the API is the same.

First, “slab” has become a generic name referring to a memory allocation strategy employing an object cache, enabling efficient allocation and deallocation of kernel objects. It was first documented by Sun engineer Jeff Bonwick1 and implemented in the Solaris 2.4 kernel.

Linux currently offers three choices for its “slab” allocator:

Slab is the original, based on Bonwick’s seminal paper and available since Linux kernel version 2.2. It is a faithful implementation of Bonwick’s proposal, augmented by the multiprocessor changes described in Bonwick’s follow-up paper2.

Slub is the next-generation replacement memory allocator, which has been the default in the Linux kernel since 2.6.23. It continues to employ the basic “slab” model, but fixes several deficiencies in Slab’s design, particularly around systems with large numbers of processors. Slub is simpler than Slab.

SLOB (Simple List Of Blocks) is a memory allocator optimized for embedded systems with very little memory—on the order of megabytes. It applies a very simple first-fit algorithm on a list of blocks, not unlike the old K&R-style heap allocator. In eliminating nearly all of the overhad from the memory allocator, SLOB is a good fit for systems under extreme memory constraints, but it offers none of the benefits described in 1 and can suffer from pathological fragmentation.

What should you use? Slub, unless you are building a kernel for an embedded device with limited in memory. In that case, I would benchmark Slub versus SLOB and see what works best for your workload. There is no reason to use Slab; it will likely be removed from future Linux kernel releases.

SLUB (software)

SLUB (the unqueued slab allocator[1]) is a memory management mechanism intended for the efficient memory allocation of kernel objects which displays the desirable property of eliminating fragmentation caused by allocations and deallocations. The technique is used to retain allocated memory that contains a data object of a certain type for reuse upon subsequent allocations of objects of the same type. It is used in Linux and became the default allocator since 2.6.23.[2]

See also [ edit ]

Notes [ edit ]

Short users guide for SLUB — The Linux Kernel documentation

The basic philosophy of SLUB is very different from SLAB. SLAB requires rebuilding the kernel to activate debug options for all slab caches. SLUB always includes full debugging but it is off by default. SLUB can enable debugging only for selected slabs in order to avoid an impact on overall system performance which may make a bug more difficult to find.

In order to switch debugging on one can add an option slub_debug to the kernel command line. That will enable full debugging for all slabs.

Typically one would then use the slabinfo command to get statistical data and perform operation on the slabs. By default slabinfo only lists slabs that have data in them. See “slabinfo -h” for more options when running the command. slabinfo can be compiled with

Some of the modes of operation of slabinfo require that slub debugging be enabled on the command line. F.e. no tracking information will be available without debugging on and validation can only partially be performed if debugging was not switched on.

Some more sophisticated uses of slub_debug:¶

Parameters may be given to slub_debug . If none is specified then full debugging is enabled. Format:

slub_debug= Enable options for all slabs slub_debug=,,,… Enable options only for select slabs (no spaces after a comma)

Possible debug options are:

F Sanity checks on (enables SLAB_DEBUG_CONSISTENCY_CHECKS Sorry SLAB legacy issues) Z Red zoning P Poisoning (object and padding) U User tracking (free and alloc) T Trace (please only use on single slabs) A Toggle failslab filter mark for the cache O Switch debugging off for caches that would have caused higher minimum slab orders – Switch all debugging off (useful if the kernel is configured with CONFIG_SLUB_DEBUG_ON)

F.e. in order to boot just with sanity checks and red zoning one would specify:

slub_debug=FZ

Trying to find an issue in the dentry cache? Try:

slub_debug=,dentry

to only enable debugging on the dentry cache. You may use an asterisk at the end of the slab name, in order to cover all slabs with the same prefix. For example, here’s how you can poison the dentry cache as well as all kmalloc slabs:

Red zoning and tracking may realign the slab. We can just apply sanity checks to the dentry cache with:

slub_debug=F,dentry

Debugging options may require the minimum possible slab order to increase as a result of storing the metadata (for example, caches with PAGE_SIZE object sizes). This has a higher liklihood of resulting in slab allocation errors in low memory situations or if there’s high fragmentation of memory. To switch off debugging for such caches by default, use:

slub_debug=O

In case you forgot to enable debugging on the kernel command line: It is possible to enable debugging manually when the kernel is up. Look at the contents of:

/sys/kernel/slab//

Look at the writable files. Writing 1 to them will enable the corresponding debug option. All options can be set on a slab that does not contain objects. If the slab already contains objects then sanity checks and tracing may only be enabled. The other options may cause the realignment of objects.

Careful with tracing: It may spew out lots of information and never stop if used on the wrong slab.

Slab merging¶ If no debug options are specified then SLUB may merge similar slabs together in order to reduce overhead and increase cache hotness of objects. slabinfo -a displays which slabs were merged together.

Slab validation¶ SLUB can validate all object if the kernel was booted with slub_debug. In order to do so you must have the slabinfo tool. Then you can do slabinfo -v which will test all objects. Output will be generated to the syslog. This also works in a more limited way if boot was without slab debug. In that case slabinfo -v simply tests all reachable objects. Usually these are in the cpu slabs and the partial slabs. Full slabs are not tracked by SLUB in a non debug situation.

Getting more performance¶ To some degree SLUB’s performance is limited by the need to take the list_lock once in a while to deal with partial slabs. That overhead is governed by the order of the allocation for each slab. The allocations can be influenced by kernel parameters: slub_min_objects allows to specify how many objects must at least fit into one slab in order for the allocation order to be acceptable. In general slub will be able to perform this number of allocations on a slab without consulting centralized resources (list_lock) where contention may occur. slub_min_order specifies a minim order of slabs. A similar effect like slub_min_objects . slub_max_order specified the order at which slub_min_objects should no longer be checked. This is useful to avoid SLUB trying to generate super large order pages to fit slub_min_objects of a slab cache with large object sizes into one high order page. Setting command line parameter debug_guardpage_minorder=N (N > 0), forces setting slub_max_order to 0, what cause minimum possible order of slabs allocation.

SLUB Debug output¶ Here is a sample of slub debug output: ==================================================================== BUG kmalloc-8: Redzone overwritten ——————————————————————– INFO: 0xc90f6d28-0xc90f6d2b. First byte 0x00 instead of 0xcc INFO: Slab 0xc528c530 flags=0x400000c3 inuse=61 fp=0xc90f6d58 INFO: Object 0xc90f6d20 @offset=3360 fp=0xc90f6d58 INFO: Allocated in get_modalias+0x61/0xf5 age=53 cpu=1 pid=554 Bytes b4 0xc90f6d10: 00 00 00 00 00 00 00 00 5a 5a 5a 5a 5a 5a 5a 5a ……..ZZZZZZZZ Object 0xc90f6d20: 31 30 31 39 2e 30 30 35 1019.005 Redzone 0xc90f6d28: 00 cc cc cc . Padding 0xc90f6d50: 5a 5a 5a 5a 5a 5a 5a 5a ZZZZZZZZ [] dump_trace+0x63/0x1eb [] show_trace_log_lvl+0x1a/0x2f [] show_trace+0x12/0x14 [] dump_stack+0x16/0x18 [] object_err+0x143/0x14b [] check_object+0x66/0x234 [] __slab_free+0x239/0x384 [] kfree+0xa6/0xc6 [] get_modalias+0xb9/0xf5 [] dmi_dev_uevent+0x27/0x3c [] dev_uevent+0x1ad/0x1da [] kobject_uevent_env+0x20a/0x45b [] kobject_uevent+0xa/0xf [] store_uevent+0x4f/0x58 [] dev_attr_store+0x29/0x2f [] sysfs_write_file+0x16e/0x19c [] vfs_write+0xd1/0x15a [] sys_write+0x3d/0x72 [] sysenter_past_esp+0x5f/0x99 [] 0xb7f7b410 ======================= FIX kmalloc-8: Restoring Redzone 0xc90f6d28-0xc90f6d2b=0xcc If SLUB encounters a corrupted object (full detection requires the kernel to be booted with slub_debug) then the following output will be dumped into the syslog: Description of the problem encountered This will be a message in the system log starting with: =============================================== BUG : ———————————————– INFO: – INFO: Slab

INFO: Object