Re: ENOSPC returned during writepages

[Andreas Dilger <adilger@sun.com>]

On Aug 20, 2008  07:53 -0400, Theodore Ts'o wrote:
> Also, this is one of the places where it might help if we did
> something like:
> 
> 	freeblocks = percpu_counter_read(&sbi->s_freeblocks_counter);
> 	if (freeblocks < NR_CPUS*4)
> 		freeblocks = percpu_counter_sum(&sbi->s_freeblocks_counter);
>
> 	if (freeblocks < total) {
> 		spin_unlock(&EXT4_I(inode)->i_block_reservation_lock);
> 		return -ENOSPC;
> 	}

This is definitely a start. Lustre does the freeblocks granting to clients
in a manner that the amount of grant is some fraction of the remaining free
space (up to a maximum), and the clients only have to block and do sync IO
when the free space is very low.

The per-CPU allocation is very similar to having multiple clients...
I don't think NR_CPUS*4 is big enough to avoid the races though.  It
needs to be something like 4 * max(FBC_BATCH, total) * NR_CPUS.

What I think makes sense, however, is that if freeblocks < $threshold that
a global spinlock is taken and the percpu_counter_sum() is done under the
lock before deciding if enough space is left.  Since it is impossible that
the other CPUs get below -FBC_BATCH away from the correct free space they
should all get the spinlock at the same time when we get too low.

> BTW, I was looking at the percpu_counter interface, and I'm confused
> why we have percpu_counter_sum_and_set() and percpu_counter_sum().  If
> we're taking the fbc->lock to calculate the precise value of the
> counter, why not simply set fbc->count?  
> 
> Also, it is singularly unfortunate that certain interfaces, such as
> percpu_counter_sum_and_set() only exist for CONFIG_SMP.  This is
> definitely post-2.6.27, but it seems to me that we probably want
> something like percpu_counter_compare_lt() which does something like this:
> 
> static inline int percpu_counter_compare_lt(struct percpu_counter *fbc,
> 					    s64 amount)
> {
> #ifdef CONFIG_SMP
> 	if ((fbc->count - amount) < FBC_BATCH)
> 		percpu_counter_sum_and_set(fbc);
> #endif
> 	return 	(fbc->count < amount);
> }
> 
> ... which we would then use in ext4_has_free_blocks() and
> ext4_da_reserve_space().
> 
> 						- Ted
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Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.