Re: ReiserFS v3 choking when free space falls below 10%?

[Jeff Mahoney <jeffm@suse.com>]

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Jan Kara wrote:
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>> Jan Kara wrote:
>>>   Hi,
>>>
>>>   just one note: I've looked to the in scan_bitmap() in bitmap.c. There is:
>>>         /* When the bitmap is more than 10% free, anyone can allocate.
>>>          * When it's less than 10% free, only files that already use the
>>>          * bitmap are allowed. Once we pass 80% full, this restriction
>>>          * is lifted.
>>>          *
>>>          * We do this so that files that grow later still have space
>>>          * close to
>>>          * their original allocation. This improves locality, and
>>>          * presumably
>>>          * performance as a result.
>>>          *
>>>          * This is only an allocation policy and does not make up for
>>>          * getting a
>>>          * bad hint. Decent hinting must be implemented for this to work
>>>          * well.
>>>          */
>>>         if (TEST_OPTION(skip_busy, s)
>>>             && SB_FREE_BLOCKS(s) > SB_BLOCK_COUNT(s) / 20) {
>>>
>>>    So the comment suggests we should lift the restriction when we are 80%
>>> full but if you see the condition, it checks wherher we are 95% full! I
>>> guess that is really asking for trouble and could explain the
>>> behaviour...
>>>   Mike could you try changing that 20 in the test to 5? IMHO that could
>>> fix your problem.
>> Shoot. I guess I never sent that mail out last night. I had discovered
>> the same thing. The thing is, I don't think it will cause the kind of
>> performance problem we're seeing here. Once it sees the 90% check it
>> will bail out. Minor slowdown, not anything like we're seeing.
>   Hmm, right. You'll only scan that one bitmap the file is in, won't
> you? That can still take some time so maybe it's worth trying this fix
> anyway.

Oh, I agree that it's a bug that needs to be fixed. I just don't think
it's causing 90% CPU usage. :)

- -Jeff

- --
Jeff Mahoney
SUSE Labs
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