Re: [PATCHv2 0/3] Allow ZRAM to use any zpool-compatible backend

[Minchan Kim]

On Thu, Dec 19, 2019 at 03:19:28PM +0100, Vitaly Wool wrote:
> The coming patchset is a new take on the old issue: ZRAM can currently be
> used only with zsmalloc even though this may not be the optimal
> combination for some configurations. The previous (unsuccessful) attempt
> dates back to 2015 [1] and is notable for the heated discussions it has
> caused.
> 
> This patchset addresses the increasing demand to deploy ZRAM in systems
> where zsmalloc is not a perfect match or is not applicable at all. An
> example of a system of the first type is an embedded system using ZRAM
> block device as a swap where quick application launch is critical for
> good user experience since z3fold is substantially faster on read than
> zsmalloc [2].

Look https://lkml.org/lkml/2019/10/29/1169

z3fold read is 15% faster *only* when when compression ratio is bad below 50%
since zsmalloc involves copy operation if the object is located in the
boundary of discrete pages. It's never popular workload.
Even, once write is involved(write-only, mixed read-write), z3fold is always
slow. Think about that swap-in could cause swap out in the field because devices
are usually under memory pressure. Also, look at the memory usage.
zsmalloc saves bigger memory for all of compression ratios. 

You claims flexibility - some user want fast read. How do you guarantee
it is always fast? It depends on compression ratio. How can admin estimate
runtime data compression ratio in advance?  Their workload is read-only
if they use zram as swap? they are okay to lose write performance and memory
efficiency? Considering that, it's niche. I don't think it's worth to add
maintenance burden here for very limited usecase.

Think about why we replaced xvmalloc with zsmalloc instead of creating wrapper
to keep two allocators and why people push back so hard when we introduced even
zsmalloc. Why zbud was born at the cost of sacrificing memory efficiency was
concern about memory fragmenation of zsmalloc so freeing memory cannot make real
free memory so wanted to manage fragmentation limit(However, we introduced
object migration in zsmalloc afterward so the concern was gone now).

> 
> A system of the second type would, for instance, be the one with
> hardware on-the-fly RAM compression/decompression where the saved RAM
> space could be used for ZRAM but would require a special allocator.

I agree. For the special compressor, we would need other allocator, even
huge zram changes in future for best performance. However, I'm not sure
zpool is good fit for the case. We need discussion when that kinds of
requirments comes up.

Nacked-by: Minchan Kim <minchan@xxxxxxxxxx>