Re: [PATCH v3 23/24] erofs: introduce cached decompression

[David Sterba]

On Mon, Jul 22, 2019 at 06:58:59PM +0800, Gao Xiang wrote:
> On 2019/7/22 ????6:18, David Sterba wrote:
> > On Mon, Jul 22, 2019 at 10:50:42AM +0800, Gao Xiang wrote:
> >> +choice
> >> +	prompt "EROFS Data Decompression mode"
> >> +	depends on EROFS_FS_ZIP
> >> +	default EROFS_FS_ZIP_CACHE_READAROUND
> >> +	help
> >> +	  EROFS supports three options for decompression.
> >> +	  "In-place I/O Only" consumes the minimum memory
> >> +	  with lowest random read.
> >> +
> >> +	  "Cached Decompression for readaround" consumes
> >> +	  the maximum memory with highest random read.
> >> +
> >> +	  If unsure, select "Cached Decompression for readaround"
> >> +
> >> +config EROFS_FS_ZIP_CACHE_DISABLED
> >> +	bool "In-place I/O Only"
> >> +	help
> >> +	  Read compressed data into page cache and do in-place
> >> +	  I/O decompression directly.
> >> +
> >> +config EROFS_FS_ZIP_CACHE_READAHEAD
> >> +	bool "Cached Decompression for readahead"
> >> +	help
> >> +	  For each request, it caches the last compressed page
> >> +	  for further reading.
> >> +	  It still does in-place I/O for the rest compressed pages.
> >> +
> >> +config EROFS_FS_ZIP_CACHE_READAROUND
> >> +	bool "Cached Decompression for readaround"
> >> +	help
> >> +	  For each request, it caches the both end compressed pages
> >> +	  for further reading.
> >> +	  It still does in-place I/O for the rest compressed pages.
> >> +
> >> +	  Recommended for performance priority.
> > 
> > The number of individual Kconfig options is quite high, are you sure you
> > need them to be split like that?
> 
> You mean the above? these are 3 cache strategies, which impact the
> runtime memory consumption and performance. I tend to leave the above
> as it-is...

No, I mean all Kconfig options, they're scattered over several patches,
best seen in the checked out branch. The cache strategies are actually
just one config option (choice).

> I'm not sure vm_map_ram() is always better than vmap() for all
> platforms (it has noticeable performance impact). However that
> seems true for my test machines (x86-64, arm64).
> 
> If vm_map_ram() is always the optimal choice compared with vmap(),
> I will remove vmap() entirely, that is OK. But I am not sure for
> every platforms though.

You can select the implementation by platform, I don't know what are the
criteria like cpu type etc, but I expect it's something that can be
determined at module load time. Eventually a module parameter can be the
the way to set it.

> > And so on. I'd suggest to go through all the options and reconsider them
> > to be built-in, or runtime settings. Debugging features like the fault
> > injections could be useful on non-debugging builds too, so a separate
> > option is fine, otherwise grouping other debugging options under the
> > main EROFS_FS_DEBUG would look more logical.
> 
> The remaining one is EROFS_FS_CLUSTER_PAGE_LIMIT. It impacts the total
> size of z_erofs_pcluster structure. It's a hard limit, and should be
> configured as small as possible. I can remove it right now since multi-block
> compression is not available now. However, it will be added again after
> multi-block compression is supported.
> 
> So, How about leave it right now and use the default value?

>From the Kconfig and build-time settings perspective I think it's
misplaced. This affects testing, you'd have to rebuild and reinstall the
module to test any change, while it's "just" a number that can be either
module parameter, sysfs knob, mount option or special ioctl.

But I may be wrong, EROFS is a special purpose filesystem, so the
fine-grained build options might make sense (eg. due to smaller code).
The question should be how does each option affect typical production
build targets. Fewer is IMHO better.