Pieter de Goeje wrote:
On Wednesday 06 January 2010 22:49:44 shrivatsan wrote:
Hi,
I have configured a malloc-backed memory disk, and I mount the device on to
the file system. I write some data onto the file system. I see that the
free memory indicated by kmem_map_free goes down, and this is proportional
to the size of the data written. However, even after removing all the
data, kmem_map_free doesn't seem to go up. Its only after detaching the
memory disk does the free memory go up. May I know the reason for this
behavior?
Thanks,
-shrivatsan
Because when you "erase" something, all it does is unlink (delete the
reference to) the data. So there is currently no way the memory disk can free
the memory associated with the data. That is also why you should normally use
swap backed memory disks instead, or use tmpfs. These can return memory to
the system.
The ability of the filesystem to mark certain blocks as "erased" is important
not only for memory disks but also for solid state drives. It is a feature
UFS2 is currently lacking unfortunately.
but is being worked on
- Pieter
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