linux-kernel - Re: [PATCH 4/4] f2fs: optimize build_free

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Date:	Wed, 8 May 2013 19:50:32 +0800
From:	Haicheng Li <haicheng.li@...ux.intel.com>
To:	Jaegeuk Kim <jaegeuk.kim@...sung.com>
Cc:	linux-kernel@...r.kernel.org, Haicheng Li <haicheng.lee@...il.com>,
	linux-fsdevel@...r.kernel.org,
	linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [PATCH 4/4] f2fs: optimize build_free_nids()

On Wed, May 08, 2013 at 06:50:04PM +0900, Jaegeuk Kim wrote:
 > > Could you explain when this can happen?
> >
> > I'm thinking of this possible scenario:
> >
> > as we don't hold any spinlock to protect the context, add_free_nid() could be
> > called by other thread anytime, e.g. by the gc_thread_func() in background.
>
> The gc_thread_func() is not a proper example here though, the
> buid_free_nids() is covered by nm_i->build_lock, so build_free_nids is
> entered only one at a time.
> In addtion, build_free_nids starts with checking if (nm_i->fcnt >
> NAT_ENTRY_PER_BLOCK) in order not to be conducted repeatedely.

surely build_free_nids() itself is under well protection.
but this scenario would happen when gc_thread_func() is running in background:
        f2fs_gc()
                write_checkpoint()
                        flush_nat_entries()
                                add_free_nid()
> >
> > then nm_i->fcnt could be increased as 2 * MAX_FREE_NIDS while i < FREE_NID_PAGES.
> > Anything I misconsidered?
>
> Apart from the correctness of this behavior, I'm not sure why we should
> strictly manage this threshold value.
> Should we really need to do this?

This threshold value itself should have already be well managed in current code.

This patch is just to avoid unecessary while-loop that tries scan_nat_page() even when this threshold
has already been reached. But as I mentioned previously, it just possibly avoids "< 4" unecessary tries.

So this patch now becomes a very very trivial optimization because scan_nat_page() itself can detect out the condition.

In such case, You can *ignore* this patch:). 
Thanks for the patch review, Jaegeuk!
 
> > 
> > hmm, the pros is that this check may possibly avoid some (< 4) unnecessary while-loop,
> > the cons is that too many checks of (nm_i->fcnt > 2 * MAX_FREE_NIDS)
> > would make the code looking messy and fragmentary...
> >  
> > > >  		if (i++ == FREE_NID_PAGES)
> > > >  			break;
> > > >  	}

-haicheng
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