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vfs: vntblinit(): Rework comments on the computation of 'kern.maxvnodes'

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Remove the parts that describe what is already in the code formula.  Add
hints about which of 'physvnodes'/'virtvnodes' takes precedence, and
when 'desiredvnodes' becomes smaller than 'maxfiles'.  These were both
computed analytically and verified experimentally.

Reviewed by:    kib (older version)
MFC after:      3 days
Sponsored by:   The FreeBSD Foundation
Differential Revision:  https://reviews.freebsd.org/D50310

(cherry picked from commit bf8890c84b)
(cherry picked from commit 5b07152f3ec3eea78958796b45594c5bd96c0bfb)

Approved by:    re (cperciva)
This commit is contained in:
Olivier Certner 2025-05-12 13:20:01 +02:00
parent fbc73f17ba
commit 0b4d5c560a
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1 changed files with 9 additions and 9 deletions
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18
sys/kern/vfs_subr.c
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@ -722,15 +722,15 @@ vntblinit(void *dummy __unused)
int cpu, physvnodes, virtvnodes;
/*
* Desiredvnodes is a function of the physical memory size and the
* kernel's heap size. Generally speaking, it scales with the
* physical memory size. The ratio of desiredvnodes to the physical
* memory size is 1:16 until desiredvnodes exceeds 98,304.
* Thereafter, the
* marginal ratio of desiredvnodes to the physical memory size is
* 1:64. However, desiredvnodes is limited by the kernel's heap
* size. The memory required by desiredvnodes vnodes and vm objects
* must not exceed 1/10th of the kernel's heap size.
* 'desiredvnodes' is the minimum of a function of the physical memory
* size and another of the kernel heap size (UMA limit, a portion of the
* KVA).
*
* Currently, on 64-bit platforms, 'desiredvnodes' is set to
* 'virtvnodes' up to a physical memory cutoff of ~1674MB, after which
* 'physvnodes' applies instead. With the current automatic tuning for
* 'maxfiles' (32 files/MB), 'desiredvnodes' becomes smaller than it at
* ~5136MB.
*/
physvnodes = maxproc + pgtok(vm_cnt.v_page_count) / 64 +
3 * min(98304 * 16, pgtok(vm_cnt.v_page_count)) / 64;