On Thu, Sep 08, 2016 at 06:07:45PM +0800, Lin Feng wrote: > Hi Dave, > > Thank you for your fast reply, look beblow please. > > On 09/08/2016 05:22 AM, Dave Chinner wrote: > >On Wed, Sep 07, 2016 at 06:36:19PM +0800, Lin Feng wrote: > >>Hi all nice xfs folks, > >> > >>I'm a rookie and really fresh new in xfs and currently I ran into an > >>issue same as the following link described: > >>http://oss.sgi.com/archives/xfs/2014-04/msg00058.html > >> > >>In my box(running cephfs osd using xfs kernel 2.6.32-358) and I sum > >>all possible memory counter can be find but it seems that nearlly > >>26GB memory has gone and they are back after I echo 2 > > >>/proc/sys/vm/drop_caches, so seems these memory can be reclaimed by > >>slab. > > > >It isn't "reclaimed by slab". The XFS metadata buffer cache is > >reclaimed by a memory shrinker, which are for reclaiming objects > >from caches that aren't the page cache. "echo 2 > > >/proc/sys/vm/drop_caches" runs the memory shrinkers rather than page > >cache reclaim. Many slab caches are backed by memory shrinkers, > >which is why it is thought that "2" is "slab reclaim".... > > > >>And according to what David said replying in the list: > >.. > >>That's where your memory is - in metadata buffers. The xfs_buf slab > >>entries are just the handles - the metadata pages in the buffers > >>usually take much more space and it's not accounted to the slab > >>cache nor the page cache. > > > >That's exactly the case. > > > >> Minimum / Average / Maximum Object : 0.02K / 0.33K / 4096.00K > >> > >> OBJS ACTIVE USE OBJ SIZE SLABS OBJ/SLAB CACHE SIZE NAME > >>4383036 4383014 99% 1.00K 1095759 4 4383036K xfs_inode > >>5394610 5394544 99% 0.38K 539461 10 2157844K xfs_buf > > > >So, you have *5.4 million* active metadata buffers. Each buffer will > >hold 1 or 2 4k pages on your kernel, so simple math says 4M * 4k + > >1.4M * 8k = 26G. There's no missing counter here.... > > Does xattr contribute to such metadata buffers or there is something else? xattrs are metadata, so if they don't fit in line in the inode (typical for ceph because it uses xattrs larger than 256 bytes) then they are held in external blocks which are cached in the buffer cache. > After consulting to my teammate, who told me that in our case small files > (there are a looot, look below) always use xattr. Which means that if you have 4.4M cached inodes, you probably have ~4.4M xattr metadata buffers in cache for those inodes, too. > Another thing is do we need to export such thing or we have to make > the computation every time to figure out if we leak memory. > And more important is that seems these memory has a low priority to > be reclaimed by memory reclaim mechanism, does it due to most of the > slab objects are active? "active" slab objects simply mean they are allocated. It does not mean they are cached or imply anything else about the object's life cycle. > >> OBJS ACTIVE USE OBJ SIZE SLABS OBJ/SLAB CACHE SIZE NAME > >> 4383036 4383014 99% 1.00K 1095759 4 4383036K xfs_inode > >> 5394610 5394544 99% 0.38K 539461 10 2157844K xfs_buf > > In fact xfs eats a lot of my ram and I will never know where it goes > without diving into xfs source, at least I'm the second extreme user > ;-) > > > > >Obviously your workload is doing something extremely metadata > >intensive to have a cache footprint like this - you have more cached > >buffers than inodes, dentries, etc. That in itself is very unusual - > >can you describe what is stored on that filesystem and how large the > >attributes being stored in each inode are? > > The fs-user behavior is that ceph-osd daemon will intensively > pull/synchronize/update files from other osd when the server is up. > In our case cephfs osd stores a lot of small pictures in the > filesystem, and I do some simple analysis, there are nearly > 3,000,000 files on each disk and there are 10 such disk. > [root@wzdx49 osd.670]# find current -type f -size -512k | wc -l > 2668769 > [root@wzdx49 ~]# find /data/osd/osd.67 -type f | wc -l > 2682891 > [root@wzdx49 ~]# find /data/osd/osd.67 -type d | wc -l > 109760 Yup, that's a pretty good indication that you have a high metadata to data ratio in each filesystem, and that ceph is accessing the metadata more intensively than the data. The fact that the metadata buffer count roughly matches the cached inode count tells me that the memory reclaim code is being fairly balanced about what it reclaims under memory pressure - I think the problem here is more that you didn't know where the memory was being used than anything else.... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
