|From:||Christoph Lameter <firstname.lastname@example.org>|
|Subject:||[SLUB 0/3] SLUB: The unqueued slab allocator V5|
|Date:||Sat, 10 Mar 2007 18:10:09 -0800 (PST)|
|Cc:||email@example.com, firstname.lastname@example.org, Christoph Lameter <email@example.com>, firstname.lastname@example.org|
[PATCH] SLUB The unqueued slab allocator v4 V4->V5: - Single object slabs only for slabs > slub_max_order otherwise generate sufficient objects to avoid frequent use of the page allocator. This is necessary to compensate for fragmentation caused by frequent uses of the page allocator. We expect slabs of PAGE_SIZE from this rule since multi object slabs require uses of fields that are in use on i386 and x86_64. See the quicklist patchset for a way to fix that issue and a patch to get rid of the PAGE_SIZE special casing. - Drop pass through to page allocator due to page allocator fragmenting memory. The buffering through large order allocations is done in SLUB. Infrequent larger order allocations cause less fragmentation than frequent small order allocations. - We need to update object sizes when merging slabs otherwise kzalloc will not initialize the full object (this caused the failure on varios platforms). - Padding checks before redzone checks so that we get messages about the corruption of whole slab and not about a single object. Note that SLUB will warn on zero sized allocations. SLAB just allocates some memory. So some traces from the usb subsystem etc should be expected. Note that the definition of the return type of ksize() is currently different between mm and Linus tree. Patch is conforming to mm. V3->V4 - Rename /proc/slabinfo to /proc/slubinfo. We have a different format after all. - More bug fixes and stabilization of diagnostic functions. This seems to be finally something that works wherever we test it. - Serialize kmem_cache_create and kmem_cache_destroy via slub_lock (Adrian's idea) - Add two new modifications (separate patches) to guarantee a mininum number of objects per slab and to pass through large allocations. V2->V3 - Debugging and diagnostic support. This is runtime enabled and not compile time enabled. Runtime debugging can be controlled via kernel boot options on an individual slab cache basis or globally. - Slab Trace support (For individual slab caches). - Resiliency support: If basic sanity checks are enabled (via F f.e.) (boot option) then SLUB will do the best to perform diagnostics and then continue (i.e. mark corrupted objects as used). - Fix up numerous issues including clash of SLUBs use of page flags with i386 arch use for pmd and pgds (which are managed as slab caches, sigh). - Dynamic per CPU array sizing. - Explain SLUB slabcache flags V1->V2 - Fix up various issues. Tested on i386 UP, X86_64 SMP, ia64 NUMA. - Provide NUMA support by splitting partial lists per node. - Better Slab cache merge support (now at around 50% of slabs) - List slab cache aliases if slab caches are merged. - Updated descriptions /proc/slabinfo output This is a new slab allocator which was motivated by the complexity of the existing code in mm/slab.c. It attempts to address a variety of concerns with the existing implementation. A. Management of object queues A particular concern was the complex management of the numerous object queues in SLAB. SLUB has no such queues. Instead we dedicate a slab for each allocating CPU and use objects from a slab directly instead of queueing them up. B. Storage overhead of object queues SLAB Object queues exist per node, per CPU. The alien cache queue even has a queue array that contain a queue for each processor on each node. For very large systems the number of queues and the number of objects that may be caught in those queues grows exponentially. On our systems with 1k nodes / processors we have several gigabytes just tied up for storing references to objects for those queues This does not include the objects that could be on those queues. One fears that the whole memory of the machine could one day be consumed by those queues. C. SLAB meta data overhead SLAB has overhead at the beginning of each slab. This means that data cannot be naturally aligned at the beginning of a slab block. SLUB keeps all meta data in the corresponding page_struct. Objects can be naturally aligned in the slab. F.e. a 128 byte object will be aligned at 128 byte boundaries and can fit tightly into a 4k page with no bytes left over. SLAB cannot do this. D. SLAB has a complex cache reaper SLUB does not need a cache reaper for UP systems. On SMP systems the per CPU slab may be pushed back into partial list but that operation is simple and does not require an iteration over a list of objects. SLAB expires per CPU, shared and alien object queues during cache reaping which may cause strange hold offs. E. SLAB has complex NUMA policy layer support SLUB pushes NUMA policy handling into the page allocator. This means that allocation is coarser (SLUB does interleave on a page level) but that situation was also present before 2.6.13. SLABs application of policies to individual slab objects allocated in SLAB is certainly a performance concern due to the frequent references to memory policies which may lead a sequence of objects to come from one node after another. SLUB will get a slab full of objects from one node and then will switch to the next. F. Reduction of the size of partial slab lists SLAB has per node partial lists. This means that over time a large number of partial slabs may accumulate on those lists. These can only be reused if allocator occur on specific nodes. SLUB has a global pool of partial slabs and will consume slabs from that pool to decrease fragmentation. G. Tunables SLAB has sophisticated tuning abilities for each slab cache. One can manipulate the queue sizes in detail. However, filling the queues still requires the uses of the spin lock to check out slabs. SLUB has a global parameter (min_slab_order) for tuning. Increasing the minimum slab order can decrease the locking overhead. The bigger the slab order the less motions of pages between per CPU and partial lists occur and the better SLUB will be scaling. G. Slab merging We often have slab caches with similar parameters. SLUB detects those on boot up and merges them into the corresponding general caches. This leads to more effective memory use. About 50% of all caches can be eliminated through slab merging. This will also decrease slab fragmentation because partial allocated slabs can be filled up again. Slab merging can be switched off by specifying slub_nomerge on boot up. Note that merging can expose heretofore unknown bugs in the kernel because corrupted objects may now be placed differently and corrupt differing neighboring objects. Enable sanity checks to find those. H. Diagnostics The current slab diagnostics are difficult to use and require a recompilation of the kernel. SLUB contains debugging code that is always available (but is kept out of the hot code paths). SLUB diagnostics can be enabled via the "slab_debug" option. Parameters can be specified to select a single or a group of slab caches for diagnostics. This means that the system is running with the usual performance and it is much more likely that race conditions can be reproduced. I. Resiliency If basic sanity checks are on then SLUB is capable of detecting common error conditions and recover as best as possible to allow the system to continue. J. Tracing Tracing can be enabled via the slab_debug=T,<slabcache> option during boot. SLUB will then protocol each action on that slabcache and dump the object contents on free. K. On demand DMA cache creation. Generally DMA caches are not needed. If a kmalloc is used with __GFP_DMA then just create this single slabcache that is needed. For systems that have no ZONE_DMA requirement the support is completely eliminated. L. Performance increase Some benchmarks have shown speed improvements on kernbench in the range of 5-10%. The locking overhead of slub is based on the underlying base allocation size. If we can reliably allocate larger order pages then it is possible to increase slub performance much further. The anti-fragmentation patches may enable further performance increases. Tested on: i386 UP + SMP, x86_64 UP + SMP + NUMA emulation, IA64 NUMA + Simulator SLUB Boot options slub_nomerge Disable merging of slabs slub_min_order=x Require a minimum order for slab caches. This increases the managed chunk size and therefore reduces meta data and locking overhead. slub_min_objects=x Mininum objects per slab. Default is 8. slub_max_order=x Avoid generating slab large than order specified. slub_debug Enable all diagnostics for all caches slub_debug=<options> Enable selective options for all caches slub_debug=<o>,<cache> Enable selective options for a certain set of caches Available Debug options F Double Free checking, sanity and resiliency R Red zoning P Object / padding poisoning U Track last free / alloc T Trace all allocs / frees (only use on individual slabs). To use SLUB: Apply this patch and then select SLUB as the default slab allocator. The output of /proc/slabinfo will then change. Here is a sample (this is an UP/SMP format. The NUMA display will show on which nodes the slabs were allocated). Flags are a Cpucache Align requested A Hardware Align required C Constructor d DMA cache D Destructor F Double free checking/Sanity p Panic on failure P Poisoning r Objects are reclaimable R RCU destroy S Memory Spreading U User Tracking T Tracing Z Red Zone Thanks to Adrian Drzewiecki <email@example.com> for many ideas and spotting a lot of bugs.
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