On 07/23/2013 08:25 AM, Dave Chinner wrote:
On Mon, Jul 22, 2013 at 02:36:27PM +0200, Bernd Schubert wrote:
On 07/22/2013 02:47 AM, Dave Chinner wrote:
On Fri, Jul 19, 2013 at 09:52:00PM +0200, Bernd Schubert wrote:
Hello Ric, hi all,
On 07/12/2013 07:20 PM, Ric Wheeler wrote:
[...]
For example, we changed XFS to have it's own metdata buffer cache
reclaim mechanisms driven by a shrinker that uses prioritised cache
reclaim to ensure we reclaim less important metadata buffers before
ones that are more frequently hit (e.g. to reclaim tree leaves
before nodes and roots). This was done because the page cache based
reclaim of metadata was completely inadequate (i.e. mostly random!)
and would frequently reclaim the wrong thing and cause performance
under memory pressure to tank....
Well, especially with XFS I see reads all the time and btrace tells
me these are meta-reads. So far I didn't find a way to make XFS to
cache meta data permanenly and so far I didn't track that down any
further.
Sure. That's what *I* want to confirm - what sort of metadata is
being read. And what I see is the inode and dentry caches getting
trashed, and that results in directory reads to repopulate the
dentry cache....
For reference and without full bonnie output, with XFS I got about
800 to 1000 creates/s.
Somewhat that seems to confirm my idea not to let file systems try
to handle it themselves, but to introduce a generic way to cache
meta data.
We already have generic metadata caches - the inode and dentry
caches.
The reason some filesystems have their own caches is that the
generic caches are not always suited to the physical metadata
structure of the filesystem, and hence they have their own
multi-level caches and reclaim implementations that are more optimal
than the generic cache mechanisms.
IOWs, there isn't an "optimal" generic metadata caching mechanism
that can be implemented.
Maybe just the generic framework should be improved?
Entirely cached hash directories (16384), which are populated with
about 16 million files, so 1000 files per hash-dir.
Version 1.96 ------Sequential Create------ --------Random Create--------
fslab3 -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
files:max:min /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP
60:32:32 1702 14 2025 12 1332 4 1873 16 2047 13 1266 3
Latency 3874ms 6645ms 8659ms 505ms 7257ms 9627ms
1.96,1.96,fslab3,1,1374655110,,,,,,,,,,,,,,60,32,32,,,1702,14,2025,12,1332,4,1873,16,2047,13,1266,3,,,,,,,3874ms,6645ms,8659ms,505ms,7257ms,9627ms
Command line parameters, details of storage, the scripts you are
running, etc please. RAM as well, as 16 million files are going to
require at least 20GB RAM to fully cache...
16 million files are only laying around in the hash directories and
are not touched at all when new files are created. So I don't know
where you take 20GB from.
Each inode in memory requires between 1-1.4k of memory depending on
the filesystem they belong to. Then there's another ~200 bytes per
dentry per inode, and if the names are long enough, then another 64+
bytes for the name of the file held by the dentry. So caching 16
million inodes (directory or files) requires 15-25GB of RAM to
cache.
Yes, but the 16 million files just lay around, I don't want to cache
them. With ext4 it works fine just to cache corresponding disk directory
blocks. So if a new file is created it can lookup from these blocks that
the file does not exist.
FWIW, have you tried experimenting with
/proc/sys/vm/vfs_cache_pressure to change the ratio of metadata to
page cache reclaim? You might find that all you need to do is change
this ratio and your problem is solved.....
Did you try that with kernels < 3.11? I did and others did, see for
example here https://nf.nci.org.au/training/talks/rjh.lug2011.pdf;
In the past it did not help at all. However, and that is really good
news, with 3.11 it eventually works. Probably due to Mel patches. Thanks
Mel!
Our file names have a typical size of 21 bytes, so with a classical
ext2 layout that gives 29 bytes, with alignment that makes 32 bytes
per directory entry. Ignoring '.' and '..' we need 125000 x 4kiB
directory blocks, so about 500MB + some overhead.
If the dentry cache stays populated, then how the filesystem lays
out dirents on disk is irrelevant - you won't ever be reading them
more than once....
How does the dentry cache help you for file creates of new files? The
dentry cache cannot know if the file exists on disk or not? Or do you
want to have a negative dentry cache of all possible file name combinations?
Numbers without context or with "handwavy context" are meaningless
for the purpose of analysis and understanding.
I just wanted to show here, that creating new files introduces reads
when meta-data have been evicted from the cache and how easily that
can happen. From my point of view the hardware does not matter much
for that purpose.
In my experience, hardware always matters when you are asking
someone else to understand and reproduce your performance
problem. It's often the single most critical aspect that we need to
understand....
This was with rotating disks as typically used to store huge amounts
of HPC data. With SSDs the effect would have been smaller, but even
SSDs are not as fast as in-memory-cache lookups.
....
Our customer systems usually have >=64GiB RAM and often _less_ than
16 million files per server. But still meta-reads impact latency and
streaming performance.
.....
Please not that bonnie++ is not ideally suitable for
meta-benchmarks, but as I said above, I just wanted to demonstrate
cache evictions.
Sure. On the other hand, you're describing a well known workload and
memory pressure eviction pattern that can be entirely prevented from
userspace. Do you reuse any of the data that is streamed to disk
before it is evicted from memory by other streaming data? I suspect
the data cache hit rate for the workloads you are describing (HPC
and bulk data storage) is around 0%.
If so, why aren't you making use of fadvise(DONTNEED) to tell the
kernel it doesn't need to cache that data that is being
read/written? That will prevent streaming Io from creating memory
pressure, and that will prevent the hot data and metadata caches
from being trashed by cold streaming IO. I know of several large
scale/distributed storage server implementations that do exactly
this...
I'm afraid it is not that easy. For example we have several users
running OpenFoam over FhGFS. And while I really think that someone
should fix OpenFoams IO routines, OpenFoam as it is has cache hit of
99%. So already due to this single program I cannot simple disable
caching on the FhGFS storage side. And there are many other examples
were caching helps. And then as fadvise(DONTNEED) does not even notify
the file systems, it does not help to implement an RPC for well behaved
applications - there would be no code path to call this RPC.
I already thought some time ago to write a simple patch, but then the
FhGFS client is not in the kernel and servers are closed source, so
chances to get such a patch accepted without a user in the kernel are
almost zero.
Remember: not all IO problems need to be solved by changing kernel
code ;)
Yes sure, therefore I'm working on different fhgfs storage layout to
allow better caching. But I think it still would be useful if file
systems could use a more suitable generic framework for caching their
metadata and if admins would have better control over that.
Cheers,
Bernd
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>