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Date: Sat, 11 Jan 2003 03:35:51 -0800
From: David Schultz <dschultz@uclink.Berkeley.EDU>
To: phk@FreeBSD.ORG
Cc: Archie Cobbs <archie@dellroad.org>, freebsd-arch@FreeBSD.ORG
Subject: Re: Virtual memory question
Message-ID: <20030111113551.GC3961@HAL9000.homeunix.com>
Mail-Followup-To: phk@FreeBSD.ORG,
	Archie Cobbs <archie@dellroad.org>, freebsd-arch@FreeBSD.ORG
References: <200301110200.h0B20rUC024725@arch20m.dellroad.org> <3788.1042270528@critter.freebsd.dk>
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Thus spake phk@FreeBSD.ORG <phk@FreeBSD.ORG>:
> In message <200301110200.h0B20rUC024725@arch20m.dellroad.org>, Archie Cobbs wri
> tes:
> 
> >The question is: how does the performance of various FreeBSD system
> >calls (especially mmap() and munmap()) degrade when a process has
> >lots and lots of tiny regions mapped into memory?
> 
> Badly.  At least it used to do:  When I wrote phkmalloc(3) I tried
> using malloc instead of sbrk(2) and it suffered because the VM system
        ^^^^^^ you mean mmap(2)?
> couldn't collapse all the individual allocations (ie: N lines in
> /proc/$pid/map

The following program does 1000 mmaps of various sizes, then
prints its own memory map.  It shows that the resulting vm_map has
only a few entries in both -CURRENT and -STABLE.  The log for
src/sys/vm/vm_map.c suggests that the collapse code may have been
broken/non-optimal at several times during the past six years.

#include <sys/types.h>
#include <sys/mman.h>
#include <err.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>

#define NUM_MMAPS       1000

int main() {
    int i, psize, fd, len;
    char *p;
    char buf[20];

    psize = getpagesize();
    for (i = 0; i < NUM_MMAPS; i++) {
        p = mmap(NULL, psize * (i % 4 + 1), PROT_READ|PROT_WRITE,
                 MAP_ANON|MAP_SHARED, -1, 0);
        if (p == NULL)
            err(1, "mmap");
    }

    snprintf(buf, 20, "/proc/%u/map", getpid());
    if ((fd = open(buf, O_RDONLY)) < 0)
        err(1, "open");
    if ((len = read(fd, p, psize - 1)) < 0)
        err(1, "read");
    p[len] = '\0';
    printf("%s\n", p);
    return 0;
}

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