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SPL(4) Kernel Interfaces Manual SPL(4) NAME spl -- parameters of the SPL kernel module DESCRIPTION spl_kmem_cache_kmem_threads=4 (uint) The number of threads created for the spl_kmem_cache task queue. This task queue is responsible for allocating new slabs for use by the kmem caches. For the majority of systems and workloads only a small number of threads are required. spl_kmem_cache_obj_per_slab=8 (uint) The preferred number of objects per slab in the cache. In general, a larger value will increase the caches memory footprint while de- creasing the time required to perform an allocation. Conversely, a smaller value will minimize the footprint and improve cache reclaim time but individual allocations may take longer. spl_kmem_cache_max_size=32 (64-bit) or 4 (32-bit) (uint) The maximum size of a kmem cache slab in MiB. This effectively limits the maximum cache object size to spl_kmem_cache_max_size/spl_kmem_cache_obj_per_slab. Caches may not be created with object sized larger than this limit. spl_kmem_cache_slab_limit=16384 (uint) For small objects the Linux slab allocator should be used to make the most efficient use of the memory. However, large objects are not supported by the Linux slab and therefore the SPL implementa- tion is preferred. This value is used to determine the cutoff be- tween a small and large object. Objects of size spl_kmem_cache_slab_limit or smaller will be allo- cated using the Linux slab allocator, large objects use the SPL al- locator. A cutoff of 16K was determined to be optimal for archi- tectures using 4K pages. spl_kmem_alloc_warn=32768 (uint) As a general rule kmem_alloc() allocations should be small, prefer- ably just a few pages, since they must by physically contiguous. Therefore, a rate limited warning will be printed to the console for any kmem_alloc() which exceeds a reasonable threshold. The default warning threshold is set to eight pages but capped at 32K to accommodate systems using large pages. This value was se- lected to be small enough to ensure the largest allocations are quickly noticed and fixed. But large enough to avoid logging any warnings when a allocation size is larger than optimal but not a serious concern. Since this value is tunable, developers are en- couraged to set it lower when testing so any new largish alloca- tions are quickly caught. These warnings may be disabled by set- ting the threshold to zero. spl_kmem_alloc_max=KMALLOC_MAX_SIZE/4 (uint) Large kmem_alloc() allocations will fail if they exceed KMALLOC_MAX_SIZE. Allocations which are marginally smaller than this limit may succeed but should still be avoided due to the ex- pense of locating a contiguous range of free pages. Therefore, a maximum kmem size with reasonable safely margin of 4x is set. kmem_alloc() allocations larger than this maximum will quickly fail. vmem_alloc() allocations less than or equal to this value will use kmalloc(), but shift to vmalloc() when exceeding this value. spl_kmem_cache_magazine_size=0 (uint) Cache magazines are an optimization designed to minimize the cost of allocating memory. They do this by keeping a per-CPU cache of recently freed objects, which can then be reallocated without tak- ing a lock. This can improve performance on highly contended caches. However, because objects in magazines will prevent other- wise empty slabs from being immediately released this may not be ideal for low memory machines. For this reason, spl_kmem_cache_magazine_size can be used to set a maximum magazine size. When this value is set to 0 the magazine size will be automatically determined based on the object size. Otherwise magazines will be limited to 2-256 objects per magazine (i.e. per CPU). Magazines may never be entirely disabled in this implementation. spl_hostid=0 (ulong) The system hostid, when set this can be used to uniquely identify a system. By default this value is set to zero which indicates the hostid is disabled. It can be explicitly enabled by placing a unique non-zero value in /etc/hostid. spl_hostid_path=/etc/hostid (charp) The expected path to locate the system hostid when specified. This value may be overridden for non-standard configurations. spl_panic_halt=0 (uint) Cause a kernel panic on assertion failures. When not enabled, the thread is halted to facilitate further debugging. Set to a non-zero value to enable. spl_taskq_kick=0 (uint) Kick stuck taskq to spawn threads. When writing a non-zero value to it, it will scan all the taskqs. If any of them have a pending task more than 5 seconds old, it will kick it to spawn more threads. This can be used if you find a rare deadlock occurs be- cause one or more taskqs didn't spawn a thread when it should. spl_taskq_thread_bind=0 (int) Bind taskq threads to specific CPUs. When enabled all taskq threads will be distributed evenly across the available CPUs. By default, this behavior is disabled to allow the Linux scheduler the maximum flexibility to determine where a thread should run. spl_taskq_thread_dynamic=1 (int) Allow dynamic taskqs. When enabled taskqs which set the TASKQ_DYNAMIC flag will by default create only a single thread. New threads will be created on demand up to a maximum allowed num- ber to facilitate the completion of outstanding tasks. Threads which are no longer needed will be promptly destroyed. By default this behavior is enabled but it can be disabled to aid performance analysis or troubleshooting. spl_taskq_thread_priority=1 (int) Allow newly created taskq threads to set a non-default scheduler priority. When enabled, the priority specified when a taskq is created will be applied to all threads created by that taskq. When disabled all threads will use the default Linux kernel thread pri- ority. By default, this behavior is enabled. spl_taskq_thread_sequential=4 (int) The number of items a taskq worker thread must handle without in- terruption before requesting a new worker thread be spawned. This is used to control how quickly taskqs ramp up the number of threads processing the queue. Because Linux thread creation and destruc- tion are relatively inexpensive a small default value has been se- lected. This means that normally threads will be created aggres- sively which is desirable. Increasing this value will result in a slower thread creation rate which may be preferable for some con- figurations. spl_taskq_thread_timeout_ms=5000 (uint) Minimum idle threads exit interval for dynamic taskqs. Smaller values allow idle threads exit more often and potentially be respawned again on demand, causing more churn. FreeBSD ports 15.quarterly May 7, 2025 SPL(4)
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