Re: [PATCH 2/2] percpu_counter: Put a reasonable upper bound on percpu_counter_batch

Hi Ted,

I understand why we would want to reduce this number.
Unfortunately, the question is what do we do if all 1024 threads try
to do buffered writes into the file system at the same instant, when
we have less than 4 megabytes of space left?

The problem is that we can then do more writes than we have space, and
we will only find out about it at write back time, when the process
may have exited already -- at which point data loss is almost
inevitable.  (We could keep the data in cache and frantically page
the system administrator to delete some files to make room for dirty
data, but that's probably not going to end well….)

What we can do if we must clamp this threshold is to also increase the
threshold at which we shift away from delayed allocation.  We'll then
allocate each block at write time, which does mean more CPU and 
less efficient allocation of blocks, but if we're down to our last 4
megabytes, there's probably not much we can do that will be efficient
as far as block layout anyway….

Thanks for the explanation, I'll go back and take another look.

Anton
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