Re: tuning/tweaking VM settings for low memory (preventing OOM)

[Willy Tarreau <w@1wt.eu>]

On Fri, Jan 12, 2007 at 03:58:08PM -0600, Kumar Gala wrote:
> I'm working on an embedded PPC setup with 64M of memory and no swap.   
> I'm trying to figure out how best to tune the VM for an OOM situation  
> I'm running into.
> 
> I'm running a 2.6.16.35 kernel and have a bittorrent app that appears  
> to be initializing a large file for it to download into.  What I see  
> before running the app:
> 
> /bigfoot/usb_disk # cat /proc/meminfo
> MemTotal:        62520 kB
> MemFree:         49192 kB
> Buffers:          8240 kB
> Cached:            740 kB
> SwapCached:          0 kB
> Active:           8196 kB
> Inactive:         1236 kB
> HighTotal:           0 kB
> HighFree:            0 kB
> LowTotal:        62520 kB
> LowFree:         49192 kB
> SwapTotal:           0 kB
> SwapFree:            0 kB
> Dirty:               0 kB
> Writeback:           0 kB
> Mapped:            916 kB
> Slab:             2224 kB
> CommitLimit:     31260 kB
> Committed_AS:     1704 kB
> PageTables:         88 kB
> VmallocTotal:   933872 kB
> VmallocUsed:      9416 kB
> VmallocChunk:   923628 kB
> 
> after the OOM:
> 
> /bigfoot/usb_disk # cat /proc/meminfo
> MemTotal:        62520 kB
> MemFree:          1608 kB
> Buffers:          8212 kB
> Cached:          42780 kB
> SwapCached:          0 kB
> Active:           6228 kB
> Inactive:        45176 kB
> HighTotal:           0 kB
> HighFree:            0 kB
> LowTotal:        62520 kB
> LowFree:          1608 kB
> SwapTotal:           0 kB
> SwapFree:            0 kB
> Dirty:           35208 kB
> Writeback:        5616 kB
> Mapped:            892 kB
> Slab:             7788 kB
> CommitLimit:     31260 kB
> Committed_AS:     1704 kB
> PageTables:         88 kB
> VmallocTotal:   933872 kB
> VmallocUsed:      9416 kB
> VmallocChunk:   923628 kB
> 
> Which makes me think that we aren't writing back fast enough.  If I  
> mount the drive "sync" the issue clearly goes away.
> 
> It appears from an strace we are doing ftruncate64(5, 178257920) when  
> we OOM.
> 
> Any ideas on VM parameters to tweak so we throttle this from occurring?

Take a look at /proc/sys/vm/bdflush. There are several useful parameters
there (doc is in linux-xxx/Documentation). For instance, the first column
is the percentage of memory used by writes before starting to write on
disk. When using tcpdump intensively, I lower this one to about 1%.

Willy