On 9/15/17, 7:53 AM, "David Sterba" <dste...@suse.cz> wrote:
> On Sun, Aug 20, 2017 at 06:38:50PM -0600, Chris Murphy wrote:
> > On Fri, Aug 18, 2017 at 10:08 AM, David Sterba <dste...@suse.cz> wrote:
> >
> > > That's quite a lot, in kernel. IIRC zlib and lzo use less than 200kb,
> > > zstd wants 800kb for level 1. And this needs to be contiguous memory, so
> > > if we're lucky and get the memory at the mount time, fine. In general
> > > the memory can be fragmented (in the worst case, there are only 4k
> > > chunks available), so we'd have to vmalloc and consume the virtual,
> > > mappings in great numbers.
> >
> > Any thoughts on bootloader support, both in general, and as it relates
> > to levels of compression and memory constraints? GRUB switches to
> > protected mode early on but other bootloaders might have more
> > limitations. I guess really it's just GRUB and extlinux right now,
> > there were patches some time ago for Das U-Boot but they still aren't
> > merged.
>
> I'm not sure if the memory requirements are same for compression and
> decompression. The table is related to compression. I've looked around
> web to find what the decompression requirements are but havne't found
> anything.
In the BtrFS implementation, the same workspaces are used both for
compression and decompression, so they allocate enough memory for
compression. Decompression requires a constant 550 KB with a 128 KB window
size (which BtrFS has). You can find the numbers using the functions
ZSTD_estimateCStreamSize() and ZSTD_estimateDStreamSize().
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