netbsd-5 deadlocks when memory is low

[Emmanuel Dreyfus <manu%netbsd.org@localhost>]

I have been dealing with a deadlock problem on netbsd-5, which I though was
related to PUFFS, but it seems it is another problem, hence the new thread.

When running perfused stress test (a build of NetBSD over a glusterfs
volume), memory gets low, and the machine hangs. I can see ioflush
is sleeping on km_getwait2 kernel memory allocation in PUFFS code:
puffs_vnop_fsync/flushvncache/puffs_vnop_strategy/puffs_msgmem_alloc
(the backtrace has more function calls, but this is to give an idea).

Indeed memory is low, but there is a lot of free swap, as DDB tells us
(see below). I understand pageademon should provide the new pages, but
it keeps sleeping on pgdaemon. 

Code from HEAD works much better: intead of freezing, it kills big
processes. It would be nice if we could reach that stage on netbsd-5: 
having a system freezing is bad. It does not even reboots on its own.

If someone has suggestions on how to improve that. For instance, why
does pagedaemon stays aslept while I have 1 free page left in memory
and 98303 left in swap.

db> show uvm                                                    
Current UVM status:
  pagesize=4096 (0x1000), pagemask=0xfff, pageshift=12
  63217 VM pages: 37237 active, 18514 inactive, 3075 wired, 1 free
  pages  44073 anon, 11651 file, 2557 exec                        
  freemin=256, free-target=341, wired-max=21072
  faults=28130665, traps=28370005, intrs=5696666, ctxswitch=12864729
  softint=10808869, syscalls=1277512298, swapins=30, swapouts=62    
  fault counts:                                                 
    noram=3, noanon=0, pgwait=0, pgrele=0
    ok relocks(total)=3868(3868), anget(retrys)=6761147(1482), amapcopy=4569671
    neighbor anon/obj pg=5435996/54403069, gets(lock/unlock)=13039755/2386     
    cases: anon=4936937, anoncow=1752606, obj=10898520, prcopy=2141234, przero=8
357613                                                                         
  daemon and swap counts:
    woke=55, revs=48, scans=55713, obscans=18396, anscans=33358
    busy=0, freed=50847, reactivate=3106, deactivate=76784     
    pageouts=4210, pending=29422, nswget=1482             
    nswapdev=2, swpgavail=98303              
    swpages=98303, swpginuse=33496, swpgonly=31129, paging=907





-- 
Emmanuel Dreyfus
manu%netbsd.org@localhost