Re: Bufferheads & page-cache reference

[Badari Pulavarty <pbadari@us.ibm.com>]

On Mon, 2005-02-14 at 14:50, William Lee Irwin III wrote:
> On Mon, Feb 14, 2005 at 01:40:58PM -0800, Andrew Morton wrote:
> >>> Seems about right.  There's also the buffer_heads_over_limit logic in
> >>> mm/vmscan.c and fs/buffer.c.  That logic has a hole in that it requires
> >>> that there be a highmem shortage before we start to reclaim the lowmem
> >>> buffer_heads, but it is somewhat helpful.
> 
> William Lee Irwin III <wli@holomorphy.com> wrote:
> >> It would be beneficial to close the hole in that logic.
> 
> On Mon, Feb 14, 2005 at 02:31:42PM -0800, Andrew Morton wrote:
> > Really?  Who's hurting?
> 
> Apart from the fact that buffer_head proliferation has been a perennial
> problem, there's little to go on. By and large 2.6.x production usage
> is not yet very significant where I can see it, where the "production"
> usage is largely more stressful on account of very long durations and
> the variety of unusual situations to which the kernel is subjected.
> 

Now that we are on the subject of bufferheads and filesystem pagecache,
the reason I am looking thro the code closely is ..

Most of DB2 customers use filesystem for their database. Under the load,
they complain that entire memory in the system is used by filesystem
pagecache, freememory is very low and system starts swapping crazy OR
see lots of memory allocation failures and OOM killer kills db2.
slabinfo shows lots of bufferheads and VM folks claim that, bufferheads
are holding a ref. on the pages, so they can't use them. So, I want
to find the truth in the story and findout what exactly happening here
and which one to blame (VM or FS or IO problems) ?

BTW, all these on 2.4 kernels and I don't have a reproducible testcase
:(

Feb 7 05:35:17 nmcopsu41 kernel: ENOMEM in do_get_write_access,
retrying.
Feb 7 05:35:18 nmcopsu41 kernel: Out of Memory: Killed process 18517
(db2sysc).
Feb 7 05:35:25 nmcopsu41 kernel: Out of Memory: Killed process 18660
(db2sysc).
Feb 7 05:35:29 nmcopsu41 kernel: Out of Memory: Killed process 18873
(db2sysc).


total used free shared buffers cached
Mem: 16304560 16284152 20408 0 228428 15093736
-/+ buffers/cache: 961988 15342572
Swap: 35655616 24448 35631168
Total: 51960176 16308600 35651576


Thanks,
Badari



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