POOL(9)

📦
version 11.0 · 2 versions
, in nixpkgs unstable

🌸 NAME 🌸

okay so this is POOL(9) from the Kernel Developer's Manual, and it's literally the resource-pool manager ✨. The whole squad is pool_init, pool_destroy, pool_get, pool_put, pool_prime, pool_sethiwat, pool_setlowat, and pool_sethardlimit — resource-pool manager. 💅

✨ SYNOPSIS ✨

#include <sys/pool.h>

void
pool_init(struct pool *pp, size_t size, u_int align, u_int align_offset,
    int flags, const char *wchan, struct pool_allocator *palloc,
    int ipl);

void
pool_destroy(struct pool *pp);

void *
pool_get(struct pool *pp, int flags);

void
pool_put(struct pool *pp, void *item);

void
pool_prime(struct pool *pp, int n);

void
pool_sethiwat(struct pool *pp, int n);

void
pool_setlowat(struct pool *pp, int n);

void
pool_sethardlimit(struct pool *pp, int n, const char *warnmess,
    int ratecap);

🛟 DESCRIPTION 🛟

These utility routines provide management of pools of fixed-sized areas of memory. Resource pools set aside an amount of memory for exclusive use by the resource pool owner. This can be used by applications to guarantee the availability of a minimum amount of memory needed to continue operation independent of the memory resources currently available from the system-wide memory allocator (malloc(9)). periodt.

🧚 INITIALIZING A POOL 🧚

The function pool_init() initializes a resource pool. The arguments are:

  • pp — The handle identifying the pool resource instance.
  • size — Specifies the size of the memory items managed by the pool.
  • align — Specifies the memory address alignment of the items returned by pool_get(). This argument must be a power of two. If zero, the alignment defaults to an architecture-specific natural alignment.
  • align_offset — The offset within an item to which the align parameter applies.
  • flags — Should be set to zero, PR_NOTOUCH, or PR_PSERIALIZE. If PR_NOTOUCH is given, free items are never used to keep internal state so that the pool can be used for non memory backed objects. If PR_PSERIALIZE is given, then the allocator guarantees that a passive serialization barrier equivalent to xc_barrier(0) will be performed before the object's backing store is returned to the system. PR_PSERIALIZE implies PR_NOTOUCH. Because of the guarantees provided by PR_PSERIALIZE, objects must never be freed to a pool using this option from either hard or soft interrupt context, as doing so may block. do not play with that one, bestie 😭
  • wchan — The ‘wait channel’ passed on to cv_wait(9) if pool_get() must wait for items to be returned to the pool.
  • palloc — Can be set to NULL or pool_allocator_kmem, in which case the default kernel memory allocator will be used. It can also be set to pool_allocator_nointr when the pool will never be accessed from interrupt context.
  • ipl — Specifies an interrupt priority level that will block all interrupt handlers that could potentially access the pool.

💀 DESTROYING A POOL 💀

The function pool_destroy() destroys a resource pool. It takes a single argument pp identifying the pool resource instance.

🎀 ALLOCATING ITEMS FROM A POOL 🎀

pool_get() allocates an item from the pool and returns a pointer to it. The arguments are:

  • pp — The handle identifying the pool resource instance.
  • flags — The flags can be used to define behaviour in case the pooled resources are depleted. If no resources are available and PR_NOWAIT is given, pool_get() returns NULL. If PR_WAITOK is given and allocation is attempted with no resources available, the function will sleep until items are returned to the pool. If both PR_LIMITFAIL and PR_WAITOK are specified, and the pool has reached its hard limit, pool_get() will return NULL without waiting, allowing the caller to do its own garbage collection; however, it will still wait if the pool is not yet at its hard limit. If the PR_ZERO flag is specified, then the memory returned will be zeroed first using memset(3).

🦋 RETURNING ITEMS TO A POOL 🦋

pool_put() returns the pool item pointed at by item to the resource pool identified by the pool handle pp. If the number of available items in the pool exceeds the maximum pool size set by pool_sethiwat() and there are no outstanding requests for pool items, the excess items will be returned to the system. The arguments to pool_put() are:

  • pp — The handle identifying the pool resource instance.
  • item — A pointer to a pool item previously obtained by pool_get().

🌈 SETTING POOL RESOURCE WATERMARKS AND LIMITS 🌈

A pool will attempt to increase its resource usage to keep up with the demand for its items. Conversely, it will return unused memory to the system should the number of accumulated unused items in the pool exceed a programmable limit.

The targets for the minimum and maximum number of free items which a pool should try to keep available are known as the high and low watermarks. The functions pool_sethiwat() and pool_setlowat() set a pool's high and low watermarks, respectively.

The limits for the minimum and maximum number of total items (both free and allocated) that the pool can have at any time are specified by the functions pool_prime() and pool_sethardlimit(), respectively. The defaults for these limits are 0 and UINT_MAX, respectively. Changing these limits will cause memory to be immediately allocated to the pool or freed from the pool as needed.

pool_sethiwat()

  • pp — The handle identifying the pool resource instance.
  • n — The maximum number of free items to keep in the pool. As items are returned and the total number of free items in the pool is larger than the maximum set by this function, any completely unused pages are released immediately. If this function is not used to specify a maximum number of items, the pages will remain associated with the pool until the system runs low on memory, at which point the VM system will try to reclaim unused pages.

pool_setlowat()

  • pp — The handle identifying the pool resource instance.
  • n — The minimum number of free items to keep in the pool. When the number of free items in the pool drops below this threshold, a non-blocking attempt is made to allocate memory for more items. The number of free items is not guaranteed to remain above this threshold.

pool_sethardlimit()

  • pp — The handle identifying the pool resource instance.
  • n — The maximum number of total items in the pool (i.e. the hard limit).
  • warnmess — The warning message that will be logged when the hard limit is reached.
  • ratecap — The minimal interval (in seconds) after which another warning message is issued when the pool hits its hard limit again.

pool_prime()

  • pp — The handle identifying the pool resource instance.
  • n — The minimum number of total items for the pool. If the current number of total items is less than the new minimum then new items are allocated with blocking allocations.

🙈 POTENTIAL PITFALLS 🙈

Note that undefined behaviour results when mixing the storage providing methods supported by the pool resource routines.

The pool resource code uses a per-pool lock to protect its internal state. If any pool functions are called in an interrupt context, the caller must block all interrupts that might cause the code to be reentered. Additionally, the functions pool_init() and pool_destroy() should never be called in interrupt context. like, ever ever 👀

🩺 DIAGNOSTICS 🩺

Pool usage logs can be enabled by defining the compile-time option POOL_DIAGNOSTIC.

🔍 CODE REFERENCES 🔍

The pool manager is implemented in the file sys/kern/subr_pool.c.

👑 SEE ALSO 👑

⭐ HISTORY ⭐

The NetBSD pool manager appeared in NetBSD 1.4.

xoxo, your Debian bestie 💖 — footer vibes: source/version is Debian, the date is April 12, 2020, and the page is POOL(9). slay ✨

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