Messages in this thread | | | Date | Thu, 12 May 2005 11:19:01 +0200 | From | Xavier Roche <> | Subject | Re: [XFS] Kernel (2.6.11) deadlock (kernel hang) in user mode when writing data through mmap on large files (64-bit systems) |
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Hi,
Christoph Hellwig wrote: > This is a known problem, or rather two of them: > 1) when dirtying lots of memory via mmap writeout happens pretty randmomly > inside the file and the filesystem has problems creating nice clusters. > In the case of extent based filesystems that's really bad as the extent > map is fragmented now > 2) XFS keeps each inode's extent map in one single memory block.
It seems that the file was really *badly* fragmented. The reason, as far as we understand the problem, was:
- a file "truncated" to _expand_ its size (using ftruncate() with a size MUCH larger that the current size, which is == 0), leading to create a "big sparse file" area - sequential write in this file (_NOT_ random) using the corresponding mmapp'ed data segment - random (!) flush from kswapd leading to allocate fragmented pages (sparse file)
The file appears to have over one million fragments:
$ xfs_bmap ./data | wc -l 1051397
$ xfs_bmap ./data | head -n 20 ./data: 0: [0..7]: 15103824..15103831 1: [8..15]: 15103840..15103847 2: [16..23]: 15103856..15103863 3: [24..31]: 15103872..15103879 4: [32..39]: 15103888..15103895 5: [40..47]: 15103904..15103911 6: [48..55]: 15103920..15103927 7: [56..63]: 15103936..15103943 8: [64..71]: 15103952..15103959 9: [72..79]: 15103968..15103975 10: [80..87]: 15104104..15104111 11: [88..95]: 15104120..15104127 12: [96..103]: 15104136..15104143 13: [104..111]: 15104152..15104159 14: [112..119]: 15104168..15104175 15: [120..127]: 15104184..15104191 16: [128..135]: 15104200..15104207 17: [136..143]: 15104216..15104223 18: [144..151]: 15104232..15104239 ..
> You're seeing allocation errors where we are trying to realloc that memory > block. > Could you try the patches that Nikita posted to -mm that should improve > this behaviour?
Well, the reasons seems to clearly be this anormal number fo fragments - is there any potential solution (in the kernel/mm), or the olny solution is a patch to ensure that ftruncate() is replaced by regulars fwrite()-zero calls ?
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