git: 4e78addbeff9 - main - buf: Make the number of pbufs slightly more dynamic

The branch main has been updated by markj:

URL: https://cgit.FreeBSD.org/src/commit/?id=4e78addbeff902aabaa87fdaafbd962f90720d69

commit 4e78addbeff902aabaa87fdaafbd962f90720d69
Author:     Mark Johnston <markj@FreeBSD.org>
AuthorDate: 2023-05-30 19:11:32 +0000
Commit:     Mark Johnston <markj@FreeBSD.org>
CommitDate: 2023-05-30 19:11:32 +0000

    buf: Make the number of pbufs slightly more dynamic
    
    Various subsystems pre-allocate a set of pbufs, allocated to implement
    I/O operations.  pbuf allocations are transient, unlike most buf
    allocations.
    
    Most subsystems preallocate nswbuf or nswbuf/2 pbufs each.  The
    preallocation ensures that pbuf allocation will succeed in low memory
    conditions, which might help avoid deadlocks.  Currently we initialize
    nswbuf = min(nbuf / 4, 256).
    
    nbuf/4 > 256 on anything but the smallest systems.  For example,
    nswbuf is 256 in a VM with 128MB of memory.  In this configuration, a
    firecracker VM with one CPU preallocates over 900 pbufs.  This consumes
    2MB of RAM and adds several milliseconds to the kernel's (very small)
    boot time.
    
    Scale nswbuf by ncpu in the common case.  I think this makes more sense
    than scaling by the amount of RAM, since pbuf allocations are transient
    and aren't used for caching.  With the change, we get nswbuf=256 with 8
    CPUs.  With fewer than 8 CPUs we'll preallocate fewer pbufs than before,
    and with more we'll preallocate more.
    
    Event:          BSDCan 2023
    Reported by:    cperciva
    Reviewed by:    glebius, kib
    MFC after:      2 months
    Differential Revision:  https://reviews.freebsd.org/D40216
---
 sys/kern/vfs_bio.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/sys/kern/vfs_bio.c b/sys/kern/vfs_bio.c
index 177b201e4819..a76ea26d8859 100644
--- a/sys/kern/vfs_bio.c
+++ b/sys/kern/vfs_bio.c
@@ -1168,7 +1168,12 @@ kern_vfs_bio_buffer_alloc(caddr_t v, long physmem_est)
 	}
 
 	if (nswbuf == 0) {
-		nswbuf = min(nbuf / 4, 256);
+		/*
+		 * Pager buffers are allocated for short periods, so scale the
+		 * number of reserved buffers based on the number of CPUs rather
+		 * than amount of memory.
+		 */
+		nswbuf = min(nbuf / 4, 32 * mp_ncpus);
 		if (nswbuf < NSWBUF_MIN)
 			nswbuf = NSWBUF_MIN;
 	}