xref: /freebsd/sys/contrib/openzfs/man/man4/spl.4 (revision bdd1243df58e60e85101c09001d9812a789b6bc4)
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15.\" Copyright 2013 Turbo Fredriksson <turbo@bayour.com>. All rights reserved.
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17.Dd August 24, 2020
18.Dt SPL 4
19.Os
20.
21.Sh NAME
22.Nm spl
23.Nd parameters of the SPL kernel module
24.
25.Sh DESCRIPTION
26.Bl -tag -width Ds
27.It Sy spl_kmem_cache_kmem_threads Ns = Ns Sy 4 Pq uint
28The number of threads created for the spl_kmem_cache task queue.
29This task queue is responsible for allocating new slabs
30for use by the kmem caches.
31For the majority of systems and workloads only a small number of threads are
32required.
33.
34.It Sy spl_kmem_cache_reclaim Ns = Ns Sy 0 Pq uint
35When this is set it prevents Linux from being able to rapidly reclaim all the
36memory held by the kmem caches.
37This may be useful in circumstances where it's preferable that Linux
38reclaim memory from some other subsystem first.
39Setting this will increase the likelihood out of memory events on a memory
40constrained system.
41.
42.It Sy spl_kmem_cache_obj_per_slab Ns = Ns Sy 8 Pq uint
43The preferred number of objects per slab in the cache.
44In general, a larger value will increase the caches memory footprint
45while decreasing the time required to perform an allocation.
46Conversely, a smaller value will minimize the footprint
47and improve cache reclaim time but individual allocations may take longer.
48.
49.It Sy spl_kmem_cache_max_size Ns = Ns Sy 32 Po 64-bit Pc or Sy 4 Po 32-bit Pc Pq uint
50The maximum size of a kmem cache slab in MiB.
51This effectively limits the maximum cache object size to
52.Sy spl_kmem_cache_max_size Ns / Ns Sy spl_kmem_cache_obj_per_slab .
53.Pp
54Caches may not be created with
55object sized larger than this limit.
56.
57.It Sy spl_kmem_cache_slab_limit Ns = Ns Sy 16384 Pq uint
58For small objects the Linux slab allocator should be used to make the most
59efficient use of the memory.
60However, large objects are not supported by
61the Linux slab and therefore the SPL implementation is preferred.
62This value is used to determine the cutoff between a small and large object.
63.Pp
64Objects of size
65.Sy spl_kmem_cache_slab_limit
66or smaller will be allocated using the Linux slab allocator,
67large objects use the SPL allocator.
68A cutoff of 16K was determined to be optimal for architectures using 4K pages.
69.
70.It Sy spl_kmem_alloc_warn Ns = Ns Sy 32768 Pq uint
71As a general rule
72.Fn kmem_alloc
73allocations should be small,
74preferably just a few pages, since they must by physically contiguous.
75Therefore, a rate limited warning will be printed to the console for any
76.Fn kmem_alloc
77which exceeds a reasonable threshold.
78.Pp
79The default warning threshold is set to eight pages but capped at 32K to
80accommodate systems using large pages.
81This value was selected to be small enough to ensure
82the largest allocations are quickly noticed and fixed.
83But large enough to avoid logging any warnings when a allocation size is
84larger than optimal but not a serious concern.
85Since this value is tunable, developers are encouraged to set it lower
86when testing so any new largish allocations are quickly caught.
87These warnings may be disabled by setting the threshold to zero.
88.
89.It Sy spl_kmem_alloc_max Ns = Ns Sy KMALLOC_MAX_SIZE Ns / Ns Sy 4 Pq uint
90Large
91.Fn kmem_alloc
92allocations will fail if they exceed
93.Sy KMALLOC_MAX_SIZE .
94Allocations which are marginally smaller than this limit may succeed but
95should still be avoided due to the expense of locating a contiguous range
96of free pages.
97Therefore, a maximum kmem size with reasonable safely margin of 4x is set.
98.Fn kmem_alloc
99allocations larger than this maximum will quickly fail.
100.Fn vmem_alloc
101allocations less than or equal to this value will use
102.Fn kmalloc ,
103but shift to
104.Fn vmalloc
105when exceeding this value.
106.
107.It Sy spl_kmem_cache_magazine_size Ns = Ns Sy 0 Pq uint
108Cache magazines are an optimization designed to minimize the cost of
109allocating memory.
110They do this by keeping a per-cpu cache of recently
111freed objects, which can then be reallocated without taking a lock.
112This can improve performance on highly contended caches.
113However, because objects in magazines will prevent otherwise empty slabs
114from being immediately released this may not be ideal for low memory machines.
115.Pp
116For this reason,
117.Sy spl_kmem_cache_magazine_size
118can be used to set a maximum magazine size.
119When this value is set to 0 the magazine size will
120be automatically determined based on the object size.
121Otherwise magazines will be limited to 2-256 objects per magazine (i.e per cpu).
122Magazines may never be entirely disabled in this implementation.
123.
124.It Sy spl_hostid Ns = Ns Sy 0 Pq ulong
125The system hostid, when set this can be used to uniquely identify a system.
126By default this value is set to zero which indicates the hostid is disabled.
127It can be explicitly enabled by placing a unique non-zero value in
128.Pa /etc/hostid .
129.
130.It Sy spl_hostid_path Ns = Ns Pa /etc/hostid Pq charp
131The expected path to locate the system hostid when specified.
132This value may be overridden for non-standard configurations.
133.
134.It Sy spl_panic_halt Ns = Ns Sy 0 Pq uint
135Cause a kernel panic on assertion failures.
136When not enabled, the thread is halted to facilitate further debugging.
137.Pp
138Set to a non-zero value to enable.
139.
140.It Sy spl_taskq_kick Ns = Ns Sy 0 Pq uint
141Kick stuck taskq to spawn threads.
142When writing a non-zero value to it, it will scan all the taskqs.
143If any of them have a pending task more than 5 seconds old,
144it will kick it to spawn more threads.
145This can be used if you find a rare
146deadlock occurs because one or more taskqs didn't spawn a thread when it should.
147.
148.It Sy spl_taskq_thread_bind Ns = Ns Sy 0 Pq int
149Bind taskq threads to specific CPUs.
150When enabled all taskq threads will be distributed evenly
151across the available CPUs.
152By default, this behavior is disabled to allow the Linux scheduler
153the maximum flexibility to determine where a thread should run.
154.
155.It Sy spl_taskq_thread_dynamic Ns = Ns Sy 1 Pq int
156Allow dynamic taskqs.
157When enabled taskqs which set the
158.Sy TASKQ_DYNAMIC
159flag will by default create only a single thread.
160New threads will be created on demand up to a maximum allowed number
161to facilitate the completion of outstanding tasks.
162Threads which are no longer needed will be promptly destroyed.
163By default this behavior is enabled but it can be disabled to
164aid performance analysis or troubleshooting.
165.
166.It Sy spl_taskq_thread_priority Ns = Ns Sy 1 Pq int
167Allow newly created taskq threads to set a non-default scheduler priority.
168When enabled, the priority specified when a taskq is created will be applied
169to all threads created by that taskq.
170When disabled all threads will use the default Linux kernel thread priority.
171By default, this behavior is enabled.
172.
173.It Sy spl_taskq_thread_sequential Ns = Ns Sy 4 Pq int
174The number of items a taskq worker thread must handle without interruption
175before requesting a new worker thread be spawned.
176This is used to control
177how quickly taskqs ramp up the number of threads processing the queue.
178Because Linux thread creation and destruction are relatively inexpensive a
179small default value has been selected.
180This means that normally threads will be created aggressively which is
181desirable.
182Increasing this value will
183result in a slower thread creation rate which may be preferable for some
184configurations.
185.
186.It Sy spl_max_show_tasks Ns = Ns Sy 512 Pq uint
187The maximum number of tasks per pending list in each taskq shown in
188.Pa /proc/spl/taskq{,-all} .
189Write
190.Sy 0
191to turn off the limit.
192The proc file will walk the lists with lock held,
193reading it could cause a lock-up if the list grow too large
194without limiting the output.
195"(truncated)" will be shown if the list is larger than the limit.
196.El
197