Re: 2.4.0-test10 Sluggish After Load

[Christoph Rohland <cr@sap.com>]

Hi Rik,

Rik van Riel <riel@conectiva.com.br> writes:
> Indeed, shared memory performance still sucks rocks.

No, it's not a performance problem. It is a hard lockup problem on
highmem machines.

I do see two problems here:
1) shm_swap_core does not handle the failure of prepare_higmem_swapout
   right and basically cannot do so. It gets called zone independant
   and should probably get called per zone. At least it has to react:
   If the lowmem zone is full it should not try to swap out highmem
   pages. I have a little workaround for that. Unfortunately this is
   not the whole thing:
2) Apparently the vm does not handle the case, where it has hardly any
   pages in the queues. My machine locks up with the following memory
   numbers:

SysRq: Show Memory
Mem-info:
Free pages:        2560kB (  1028kB HighMem)
( Active: 5, inactive_dirty: 27, inactive_clean: 27, free: 640 (638 1276 1914) )0*4kB 0*8kB 0*16kB 0*32kB 0*64kB 0*128kB 0*256kB 1*512kB 0*1024kB 0*2048kB = 512kB)
7*4kB 14*8kB 1*16kB 1*32kB 1*64kB 0*128kB 1*256kB 1*512kB 0*1024kB 0*2048kB = 1020kB)
3*4kB 1*8kB 1*16kB 1*32kB 1*64kB 1*128kB 1*256kB 1*512kB 0*1024kB 0*2048kB = 1028kB)
Swap cache: add 31231, delete 31218, find 21433/21531
Free swap:       1961028kB
2162688 pages of RAM
1867776 pages of HIGHMEM
102296 reserved pages
1204551 pages shared
13 pages swap cached
0 pages in page table cache
Buffer memory:      100kB
    CLEAN: 2 buffers, 8 kbyte, 2 used (last=2), 0 locked, 0 protected, 0 dirty
   LOCKED: 25 buffers, 100 kbyte, 25 used (last=25), 2 locked, 0
   protected, 0 dirty  

You see: you only have 5+27+27=59 pages under your control...

> I have not had time to fix this and I'm afraid I probably
> won't have time to fix this in the near future. I'm willing
> to give some advice to people who do want to take on the job
> of fixing SHM performance, though..

As you know I started this some time before, but nobody helped me in
integrating it into the rest of the VM :-( 

Unfortunately my time is very limited, but I probably would give it a
second try since I think shm swapping should work. But I think a clean
integration is a 2.5 issue.

Greetings
                Christoph

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