[00/36] Large Blocksize Support V6

clameter
2007-08-28T19:09:00Z

[An update before the Kernel Summit because of the numerous requests that I
have had for this patchset. Please speak up if you feel that we need something
like this.]

This patchset modifies the Linux kernel so that larger block sizes than
page size can be supported. Larger block sizes are handled by using
compound pages of an arbitrary order for the page cache instead of
single pages with order 0.

Support is added in a way that limits the changes to existing code.
As a result filesystems can support I/O using large buffers with minimal
changes.

The page cache functions are mostly unchanged. Instead of a page struct
representing a single page they take a head page struct (which looks
the same as a regular page struct apart from the compound flags) and
operate on those. Most page cache functions can stay as they are.

No locking protocols are added or modified.

The support is also fully transparent at the level of the OS. No
specialized heuristics are added to switch to larger pages. Large
page support is enabled by filesystems or device drivers when a device
or volume is mounted. Larger block sizes are usually set during volume
creation although the patchset supports setting these sizes per file.
The formattted partition will then always be accessed with the
configured blocksize.

Some of the changes are:

- Replace the use of PAGE_CACHE_XXX constants to calculate offsets into
pages with functions that do the the same and allow the constants to
be parameterized.

- Extend the capabilities of compound pages so that they can be
put onto the LRU and reclaimed.

- Allow setting a larger blocksize via set_blocksize()

Rationales:
-----------

1. The ability to handle memory of an arbitrarily large size using
a singe page struct "handle" is essential for scaling memory handling
and reducing overhead in multiple kernel subsystems. This patchset
is a strategic move that allows performance gains throughout the
kernel.

2. Reduce fsck times. Larger block sizes mean faster file system checking.
Using 64k block size will reduce the number of blocks to be managed
by a factor of 16 and produce much denser and contiguous metadata.

3. Performance. If we look at IA64 vs. x86_64 then it seems that the
faster interrupt handling on x86_64 compensate for the speed loss due to
a smaller page size (4k vs 16k on IA64). Supporting larger block sizes
sizes on all allows a significant reduction in I/O overhead and increases
the size of I/O that can be performed by hardware in a single request
since the number of scatter gather entries are typically limited for
one request. This is going to become increasingly important to support
the ever growing memory sizes since we may have to handle excessively
large amounts of 4k requests for data sizes that may become common
soon. For example to write a 1 terabyte file the kernel would have to
handle 256 million 4k chunks.

4. Cross arch compatibility: It is currently not possible to mount
an 16k blocksize ext2 filesystem created on IA64 on an x86_64 system.
With this patch this becomes possible. Note that this also means that
some filesystems are already capable of working with blocksizes of
up to 64k (ext2, XFS) which is currently only available on a select
few arches. This patchset enables that functionality on all arches.
There are no special modifications needed to the filesystems. The
set_blocksize() function call will simply support a larger blocksize.

5. VM scalability
Large block sizes mean less state keeping for the information being
transferred. For a 1TB file one needs to handle 256 million page
structs in the VM if one uses 4k page size. A 64k page size reduces
that amount to 16 million. If the limitation in existing filesystems
are removed then even higher reductions become possible. For very
large files like that a page size of 2 MB may be beneficial which
will reduce the number of page struct to handle to 512k. The variable
nature of the block size means that the size can be tuned at file
system creation time for the anticipated needs on a volume.

6. IO scalability
The IO layer will receive large blocks of contiguious memory with
this patchset. This means that less scatter gather elements are needed
and the memory used is guaranteed to be contiguous. Instead of having
to handle 4k chunks we can f.e. handle 64k chunks in one go.

7. Limited scatter gather support restricts I/O sizes.

A lot of I/O controllers are limited in the number of scatter gather
elements that they support. For example a controller that support 128
entries in the scatter gather lists can only perform I/O of 128*4k =
512k in one go. If the blocksize is larger (f.e. 64k) then we can perform
larger I/O transfers. If we support 128 entries then 128*64k = 8M
can be transferred in one transaction.

Dave Chinner measured a performance increase of 50% when going to 64k
blocksize with XFS with an earlier version of this patchset.

8. We have problems supporting devices with a higher blocksize than
page size. This is for example important to support CD and DVDs that
can only read and write 32k or 64k blocks. We currently have a shim
layer in there to deal with this situation which limits the speed
of I/O. The developers are currently looking for ways to completely
bypass the page cache because of this deficiency.

9. 32/64k blocksize is also used in flash devices. Same issues.

10. Future harddisks will support bigger block sizes that Linux cannot
support since we are limited to PAGE_SIZE. Ok the on board cache
may buffer this for us but what is the point of handling smaller
page sizes than what the drive supports?

Acceptance issues:
------------------

The patchset is a pretty significant change to the way that the Linux kernel
operates. I have tried to keep the changes as minimal as possible and I
believe that this is a reasonable start to introduce large block I/O
capabilities.

The Linux VM is gradually acquiring abilities to defragment memory. These
capabilities are partially present for 2.6.23. Later versions may merge
more of the defragmentation work. The use of large pages may cause
significant fragmentation to memory. Without proper defragmentation support
these patches cannot work reliably and may cause OOMs (although I have
rarely seen those and I have seen none with 2.6.23 when testing with 16k
blocksize. Possibly the effect of the limited defragmentation capabilities
in 2.6.23 is already sufficient. Beware: Larger blocksize can cause
more defragmentation).

A number of key developers are hesitant about this functionality given the
problems that we have had in the past with memory defragmentation and the
invasiveness of the patchset. It is to be expected that it will take awhile
until confidence in the defragmentation logic builds up. So the patchset
may still have to exist for a long time unmerged. However, it is necessary
that work on this patchset continue even outside of the kernel tree in order
to mature this patchset until it can be merged at some point in the future.

The most serious shortcoming of this patchset is the lack of mmap support.
This means f.e. that it is not possible to execute binaries. Adding
mmap support would mean more changes to the VM. In order to do that some
support by multiple developers is likely needed. The current idea is to
allow the mapping of 4K segments of larger pages to preserve mmap semantics.
This means that application programs could still map pages in 4k chunks and
the VM would provide a mapping into a subsection of a larger page.

How to make this patchset work:
-------------------------------

1. Apply this patchset or do a

git pull
git://git.kernel.org/pub/scm/linux/kernel/git/christoph/largeblocksize.git
largeblock

(The git archive is used to keep the patchset up to date. Please send patches
against the git tree)

2. Enable LARGE_BLOCKSIZE Support
3. Compile kernel

In order to use a filesystem with a larger blocksize it needs to be formatted
for that larger blocksize. This is done using the mkfs.xxx tool for each
filesystem. Surprisingly the existing tools work without modification. These
formatting tools may warn you that the blocksize you specify is not supported
on your particular architecture. Ignore that warning since this is no longer
true after you have applied this patchset.

Tested file systems:

Filesystem Max Blocksize Changes

Reiserfs 8k Page size functions
Ext2 64k Page size functions
XFS 64k Page size functions / Remove PAGE_SIZE check
Ramfs MAX_ORDER Parameter to specify order

Todo/Issues:

- There are certainly numerous issues with this patch. I have only tested
copying files back and forth, volume creation etc. Others have run
fsxlinux on the volumes. The missing mmap support limits what can be
done for now.

- ZONE_MOVABLE is available in 2.6.23. Using the kernelcore=xxx as a kernel
parameter enables an area where defragmentation can work. This may be
necessary to avoid OOMs although I have seen no problems with up to 32k
blocksize even without that measure.

- The antifragmentation patches in Andrew's tree address more fragmentation
issues. However, large orders may still lead to fragmentation
of the movable sections. Memory compaction is still not merged and will
likely be needed to reliably support even larger orders of 256k or more.
How memory compaction impacts performance still has to be determined.

- Support for bouncing pages.

- Remove PAGE_CACHE_xxx constants after using page_cache_xxx functions
everywhere. But that will have to wait until merging becomes possible.
For now certain subsystems (shmem f.e.) are not using these functions.
They will only use order 0 pages.

- Support for non harddisk based filesystems. Remove the pktdvd etc
layers needed because the VM current does not support sufficiently
large blocksizes for these devices. Look for other places in the kernel
where we have similar issues.

- Mmap support

V5->V6:
- Rediff against 2.6.23-rc4
- Fix breakage introduced by updates to reiserfs
- Readahead fixes by Fengguang Wu <fengguang.wu@gmail.com>
- Provide a git tree that is kept up to date

V4->V5:
- Diff against 2.6.22-rc6-mm1
- provide test tree on ftp.kernel.org:/pub/linux

V3->V4
- It is possible to transparently make filesystems support larger
blocksizes by simply allowing larger blocksizes in set_blocksize.
Remove all special modifications for mmap etc from the filesystems.
This now makes 3 disk based filesystems that can use larger blocks
(reiser, ext2, xfs). Are there any other useful ones to make work?
- Patch against 2.6.22-rc4-mm2 which allows the use of Mel's antifrag
logic to avoid fragmentation.
- More page cache cleanup by applying the functions to filesystems.
- Disable bouncing when the gfp mask is setup.
- Disable mmap directly in mm/filemap.c to avoid filesystem changes
while we have no mmap support for higher order pages.

RFC V2->V3
- More restructuring
- It actually works!
- Add XFS support
- Fix up UP support
- Work out the direct I/O issues
- Add CONFIG_LARGE_BLOCKSIZE. Off by default which makes the inlines revert
back to constants. Disabled for 32bit and HIGHMEM configurations.
This also allows a gradual migration to the new page cache
inline functions. LARGE_BLOCKSIZE capabilities can be
added gradually and if there is a problem then we can disable
a subsystem.

RFC V1->V2
- Some ext2 support
- Some block layer, fs layer support etc.
- Better page cache macros
- Use macros to clean up code.

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