Re: JFFS2 garbage collector blocking for minutes after mount

[David Woodhouse <dwmw2@infradead.org>]

On Wed, 2005-07-20 at 11:03 +0200, Martin Egholm Nielsen wrote:
> I have a "small" NAND device (32MByte) with JFFS2 on top used as the 
> root-fs on my system. It's been working like a charm - until just now. 
> After (over)writing a relative large file (11 megs uncomp. - 
> ~3-5compr.), the garbage collector (jffs2_gcd_mtd0) uses 8:45 minutes of 
> CPU time (~99%) after booting - blocking any write operations.
> Ok, I accept that some GC'ing should be performed when going "beyond the 
> edge" - but shouldn't this be a one-time process, so the next time I 
> boot this is done with?
> I see it everytime I reboot - without touching any files on the system...
> 
> I use the mtd source from 2005-03-04...

The garbage collection thread is also responsible for building up the
node tree for every inode after mounting, so that we know for sure which
nodes are valid and which are obsolete. On NAND flash we can't actually
mark nodes as obsolete.

We've made some significant improvements to that process since March,
especially where inodes with large numbers of nodes are concerned. We
used to sort all the nodes into a linked list before building the tree,
but now we use a tree data structure for that instead -- so it's
O(n log n) instead of O(n²) in the number of nodes.

Artem has patches which should improve it still further -- I'm not sure
if they are committed yet.

-- 
dwmw2