Messages in this thread | | | Date | Mon, 22 Dec 2014 15:16:04 -0800 | From | Jaegeuk Kim <> | Subject | Re: [RFC PATCH] f2fs: add extent cache base on rb-tree |
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Hi Chao,
On Mon, Dec 22, 2014 at 03:10:30PM +0800, Chao Yu wrote: > Hi Changman, > > > -----Original Message----- > > From: Changman Lee [mailto:cm224.lee@samsung.com] > > Sent: Monday, December 22, 2014 10:03 AM > > To: Chao Yu > > Cc: Jaegeuk Kim; linux-f2fs-devel@lists.sourceforge.net; linux-kernel@vger.kernel.org > > Subject: Re: [RFC PATCH] f2fs: add extent cache base on rb-tree > > > > Hi Yu, > > > > Good approach. > > Thank you. :) > > > As you know, however, f2fs breaks extent itself due to COW. > > Yes, and sometimes f2fs use IPU when override writing, in this condition, > by using this approach we can cache more contiguous mapping extent for better > performance.
Hmm. When f2fs faces with this case, there is no chance to make an extent itself at all.
> > > Unlike other filesystem like btrfs, minimum extent of f2fs could have 4KB granularity. > > So we would have lots of extents per inode and it could lead to overhead > > to manage extents. > > Agree, the more number of extents are growing in one inode, the more memory > pressure and longer latency operating in rb-tree we are facing. > IMO, to solve this problem, we'd better to add limitation or shrink ability into > extent cache: > 1.limit extent number per inode with the value set from sysfs and discard extent > from inode's extent lru list if we touch the limitation; (e.g. in FAT, max number > of mapping extent per inode is fixed: 8) > 2.add all extents of inodes into a global lru list, we will try to shrink this list > if we're facing memory pressure. > > How do you think? or any better ideas are welcome. :)
Historically, the reason that I added only one small extent cache is that I wanted to avoid additional data structures having any overhead in critical data write path. Instead, I intended to use a well operating node page cache.
We need to consider what would be the benefit when using extent cache rather than existing node page cache.
Thanks,
> > > > > Anyway, mount option could be alternative for this patch. > > Yes, will do. > > Thanks, > Yu > > > > > On Fri, Dec 19, 2014 at 06:49:29PM +0800, Chao Yu wrote: > > > Now f2fs have page-block mapping cache which can cache only one extent mapping > > > between contiguous logical address and physical address. > > > Normally, this design will work well because f2fs will expand coverage area of > > > the mapping extent when we write forward sequentially. But when we write data > > > randomly in Out-Place-Update mode, the extent will be shorten and hardly be > > > expanded for most time as following reasons: > > > 1.The short part of extent will be discarded if we break contiguous mapping in > > > the middle of extent. > > > 2.The new mapping will be added into mapping cache only at head or tail of the > > > extent. > > > 3.We will drop the extent cache when the extent became very fragmented. > > > 4.We will not update the extent with mapping which we get from readpages or > > > readpage. > > > > > > To solve above problems, this patch adds extent cache base on rb-tree like other > > > filesystems (e.g.: ext4/btrfs) in f2fs. By this way, f2fs can support another > > > more effective cache between dnode page cache and disk. It will supply high hit > > > ratio in the cache with fewer memory when dnode page cache are reclaimed in > > > environment of low memory. > > > > > > Todo: > > > *introduce mount option for extent cache. > > > *add shrink ability for extent cache. > > > > > > Signed-off-by: Chao Yu <chao2.yu@samsung.com> > > > ---
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