RE: [f2fs-dev] [PATCH 2/2] f2fs: shrink unreferenced extent_caches first

[Chao Yu]

> -----Original Message-----
> From: Jaegeuk Kim [mailto:jaegeuk@xxxxxxxxxx]
> Sent: Saturday, July 11, 2015 7:54 AM
> To: Chao Yu; Chao Yu
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx; linux-fsdevel@xxxxxxxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx
> Subject: Re: [f2fs-dev] [PATCH 2/2] f2fs: shrink unreferenced extent_caches first
> 
> On Wed, Jul 08, 2015 at 05:41:57PM +0800, Chao Yu wrote:
> 
> [snip]
> 
> > > > > You're right. We don't need to drop it having the lowest ino first.
> > > > > Actually, I was supposed to add an LRU list for extent_trees.
> > > > > But, do we need to really take care of its order for already evicted inodes?
> > > > >
> > > > > Here, we should think about two types of hit ratios.
> > > > > One is for inodes, and the other is for data.
> > > > > The VFS maintains inode_cache in an LRU order, while its data is cached via
> > > > > page cache also conrolled by LRU. And, whenever inode is evicted, VFS drops
> > > > > all the cached data.
> > > > > So, I believe we should give a higher priority to inodes rather than data.
> > > > >
> > > > > And, in order to increase the hit ratio, we're trying to keep an extent tree
> > > > > and its nodes even if its corresponding inode was evicted.
> > > > > So, I concluded that the dropping order would not be critical especially for
> > > > > the already evicted inodes.
> > > >
> > > > >From perspective of shrinking memory size, I think you're completely right,
> > > > because we can regard extent tree and its nodes as metadata of one inode, if
> > > > VFS evict one inode, all its data and metadata include data in extent cache
> > > > should be evicted.
> > > >
> > > > But from perspective of arising hit ratio of our cache, I'm not sure this is
> > > > the best way.
> > > >
> > > > I guess in this method, we may encounter lower coverage area of dnode + extent
> > > > cache and double caches exist issue, like:
> > > > a) all data of inode is evicted by VFS, and its tree and nodes in extent cache
> > > > is evicted too, resulting lower hit raito of further access.
> > >
> > > Well, f2fs_evict_inode does not destroy whole its extent tree and nodes right
> > > away, but just drops the refcount of the extent tree. So, I expect that this
> > > gives another chance of cache hit for further data access.
> >
> > Agreed.
> >
> > > Moreover, since this only matters with memory pressure, the unreferenced extent
> > > trees and nodes would be kept long enough beyond the normal situation.
> >
> > I'm worry about the 'only matters' thing, I will investigate it if I have time.
> >
> > >
> > > > b) data and node cache of inode is exist in VFS, and its tree and nodes in extent
> > > > cache is exist too.
> > >
> > > We know that this is a separate issue, since there is no such code to check
> > > whether data and node cache exist along with extent cache entries.
> >
> > Well, just thought, for multimedia objects, like a movie file, most time
> > we will just read it, there will no further writes in it. So why not
> > building extent cache for reaccessing, and meanwhile releasing dnode pages
> > for saving memory?
> 
> Hmm, my basic approach is letting mm reclaim caches in an LRU manner as much
> as possible.
> Of course, we can think about many specific conditions, but IMO, it is not
> enough to treat them as general ones.

It's just one thought flash in my mind, I hope this can help even a litter for
memory saving in f2fs. Anyway, I agree with you, and let's stabilize and enhance
'these general ones'.

> 
> >
> > > And, I don't think we should eliminate such the duplicate, since the extent
> > > cache is a supplemenray subset of data and node caches.
> >
> > Right.
> >
> > >
> > > >
> > > > >
> > > > > >
> > > > > > And this step releasing breaks the rule of lru runs.
> > > > > >
> > > > > > Some unreferenced file has high hit ratio and some referenced file may
> > > > > > have low hit ratio. Why not release low hit ratio extent tree at first?
> > > > >
> > > > > But, still user has opened the referenced file to further access, right?
> > > >
> > > > It depends on access model.
> > > >
> > > > What I mean is that if extent cache of one inode can be hit for many times,
> > > > we can assume the access model is re-accessly, that means, we can expect
> > > > this cache can be hit more times. On the contrary, we can release it if it
> > > > is necessary.
> > >
> > > Yes, exactly it depends on user workloads.
> > >
> > > As a counter example,
> > > 1. thread A wrote extents and remained the file as it was opened to use later,
> > > 2. thread B wrote many extents newly and never touched.
> > >
> > > After #2, if shrinker was activated, the extents cached by thread A would
> > > be evicted, resulting in cache misses on further thread A's accesses.
> >
> > I didn't understand, if thread A's file is opened for reusing, from long-term
> > view, it will have high hit ratio in its extent cache than thread B's, Why
> > thread B's extent cache is not be evicted firstly.
> 
> Not long-term view. Like #1 -> #2 -> shrink -> #1 -> ...

That's reasonable, Thanks for your explanation. :)

> 
> >
> > >
> > > IMO, this can happen when a bunch of data blocks were written without updates,
> > > while some opened library/database files will access the data sooner or later.
> >
> > You mean the file with one time written in its life, like lib file or multimedia
> > file?
> >
> > So I guess what you mean is that, some app keeps file opened for long time, and
> > will access it again sooner or later, at least we should keep these referenced
> > extent cache visible rather than evicting them before evicting unreferenced one's.
> 
> Something like that. :)

OK, I can understand completely, so for now, let's do it in your way.
If any further thoughts, I will discuss with you. :)

Reviewed-by: Chao Yu <chao2.yu@xxxxxxxxxxx>

> 
> Thanks,
> 
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