Readahead for compressed data

[Qu Wenruo]

On 2021/10/22 17:11, Gao Xiang wrote:
> On Fri, Oct 22, 2021 at 10:41:27AM +0200, Jan Kara wrote:
>> On Thu 21-10-21 21:04:45, Phillip Susi wrote:
>>>
>>> Matthew Wilcox <willy at infradead.org> writes:
>>>
>>>> As far as I can tell, the following filesystems support compressed data:
>>>>
>>>> bcachefs, btrfs, erofs, ntfs, squashfs, zisofs
>>>>
>>>> I'd like to make it easier and more efficient for filesystems to
>>>> implement compressed data.  There are a lot of approaches in use today,
>>>> but none of them seem quite right to me.  I'm going to lay out a few
>>>> design considerations next and then propose a solution.  Feel free to
>>>> tell me I've got the constraints wrong, or suggest alternative solutions.
>>>>
>>>> When we call ->readahead from the VFS, the VFS has decided which pages
>>>> are going to be the most useful to bring in, but it doesn't know how
>>>> pages are bundled together into blocks.  As I've learned from talking to
>>>> Gao Xiang, sometimes the filesystem doesn't know either, so this isn't
>>>> something we can teach the VFS.
>>>>
>>>> We (David) added readahead_expand() recently to let the filesystem
>>>> opportunistically add pages to the page cache "around" the area requested
>>>> by the VFS.  That reduces the number of times the filesystem has to
>>>> decompress the same block.  But it can fail (due to memory allocation
>>>> failures or pages already being present in the cache).  So filesystems
>>>> still have to implement some kind of fallback.
>>>
>>> Wouldn't it be better to keep the *compressed* data in the cache and
>>> decompress it multiple times if needed rather than decompress it once
>>> and cache the decompressed data?  You would use more CPU time
>>> decompressing multiple times, but be able to cache more data and avoid
>>> more disk IO, which is generally far slower than the CPU can decompress
>>> the data.
>>
>> Well, one of the problems with keeping compressed data is that for mmap(2)
>> you have to have pages decompressed so that CPU can access them. So keeping
>> compressed data in the page cache would add a bunch of complexity. That
>> being said keeping compressed data cached somewhere else than in the page
>> cache may certainly me worth it and then just filling page cache on demand
>> from this data...
>
> It can be cached with a special internal inode, so no need to take
> care of the memory reclaim or migration by yourself.

There is another problem for btrfs (and maybe other fses).

Btrfs can refer to part of the uncompressed data extent.

Thus we could have the following cases:

	0	4K	8K	12K
	|	|	|	|
		    |	    \- 4K of an 128K compressed extent,
		    |		compressed size is 16K
		    \- 4K of an 64K compressed extent,
			compressed size is 8K

In above case, we really only need 8K for page cache, but if we're
caching the compressed extent, it will take extra 24K.

It's definitely not really worthy for this particular corner case.

But it would be pretty common for btrfs, as CoW on compressed data can
easily lead to above cases.

Thanks,
Qu

>
> Otherwise, these all need to be take care of. For fixed-sized input
> compression, since they are reclaimed in page unit, so it won't be
> quite friendly since such data is all coupling. But for fixed-sized
> output compression, it's quite natural.
>
> Thanks,
> Gao Xiang
>
>>
>> 								Honza
>> --
>> Jan Kara <jack at suse.com>
>> SUSE Labs, CR