Date: 27 May 2006 02:25:26 +0200 From: "Arno J. Klaassen" <arno@heho.snv.jussieu.fr> To: freebsd-current@freebsd.org Subject: indefinite wait buffer Message-ID: <wphd3c48ll.fsf@heho.labo>
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[-- Attachment #1 --]
we use FreeBSD amongst others for scientific calculations, and
ran into the 'indefinite wait buffer' problem on ordinary
swap/dump devices :
the swap-overhead being justified by enabling greater data-sets
to be treated and processes grosso-modo still being CPU-bound
rather than I/O(swap)-bound.
On recent RELENG_6 however, this fails (for sure on scrappy
ATA-devices, rather easy as well on SCSI-devices though they
seem to persist 'a couple of' 'indefinite wait buffer'
warnings).
I tested today on an amd64-notebook with 1G physmem and 4G swap
on a from-the-shelf ATA-disk.
I wrote the following code :
int
main (int argc, char **argv)
{
unsigned long maxpage;
int * base, * ptr;
_malloc_options = "AJ";
maxpage = strtol(argv[1],(char **)NULL, 10) * M_SIZE;
fprintf (stderr, "Allocing %ld Bytes\n", maxpage);
base = (int *)(malloc (maxpage));
if (base == NULL ) { fprintf (stderr, "Jammer\n"); }
while (0 == 0) {
int * ptr = base;
unsigned int i = 0;
for (i=0; i< maxpage/sizeof(int); i++) {
*(ptr++) += 1;
}
fprintf (stderr, "Loop <%d> done\n", iter);
iter++;
}
exit (0);
}
Calling this (on RELENG_6) with an argument in between
1024 and 1500 in a few minutes deadlocks the notebook
with an 'indefinite wait buffer' in /var/log/messages after reboot.
After some fiddling I came to the attached amateuristic patch :
swap_pager.c has a heuristiquely (I suppose) timeout of 20 seconds
for a msleep call; I changed this for a timeout based
on a presupposed pessimistic minimal througput for the swapping
device multiplied by the minimum of swapsize and physmem.
With this patch I can run the above code without deadlock even with
4096 (Meg) as argument..
Two remarks :
1 I abuse linux.ko/linprocfs to correctly initialise my code
2 This is no solution to the real 'indefinite wait buffer' problems
since at shutdown it still panics with 'swap_pager_force_pagein: read
from swap failed', but at least it keeps the system functional while
working.
I hope someone can comment this idea.
Best regards, Arno
[-- Attachment #2 --]
Index: sys/vm/swap_pager.c
===================================================================
RCS file: /home/ncvs/src/sys/vm/swap_pager.c,v
retrieving revision 1.273.2.2
diff -r1.273.2.2 swap_pager.c
285a286,288
> static int MB_per_sec = 1; /* be pessimist, might be sysctl'ed */
> static int timo_secs = 60; /* for msleep() in swap_pager_getpages() */
>
401a405
>
604a609
>
1104c1109
< if (msleep(mreq, &vm_page_queue_mtx, PSWP, "swread", hz*20)) {
---
> if (msleep(mreq, &vm_page_queue_mtx, PSWP, "swread", hz*timo_secs)) {
1106,1107c1111,1114
< "swap_pager: indefinite wait buffer: bufobj: %p, blkno: %jd, size: %ld\n",
< bp->b_bufobj, (intmax_t)bp->b_blkno, bp->b_bcount);
---
> "swap_pager: wait buffer timeout (%d secs): bufobj: %p, blkno: %jd, size: %ld\n",
> timo_secs, bp->b_bufobj, (intmax_t)bp->b_blkno, bp->b_bcount);
> /* wait & pray ... respect mutex */
> msleep(mreq, &vm_page_queue_mtx, PSWP, "swread", 0);
2240a2248
> int timo_secs_swap, timo_secs_physmem;
2248a2257,2265
>
> timo_secs_swap = MB_per_sec * (*total * PAGE_SIZE) / (1024*1024);
> timo_secs_physmem = MB_per_sec * (physmem * PAGE_SIZE) / (1024*1024);
> timo_secs = min(timo_secs_swap, timo_secs_physmem);
>
> if (timo_secs < 60) timo_secs=60;
> printf("ARNO timo_secs = <%d>.\n", timo_secs);
> printf("ARNO timo_secs_swap = <%d> timo_secs_physmem <%d>.\n",
> timo_secs_swap, timo_secs_physmem);
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