Re: [PATCH 5/5] btrfs: Increase buffer size for zlib functions

[David Sterba]

On Tue, Nov 26, 2019 at 10:52:49AM -0500, Josef Bacik wrote:
> On Tue, Nov 26, 2019 at 03:41:30PM +0100, Mikhail Zaslonko wrote:
> > Due to the small size of zlib buffer (1 page) set in btrfs code, s390
> > hardware compression is rather limited in terms of performance. Increasing
> > the buffer size to 4 pages would bring significant benefit for s390
> > hardware compression (up to 60% better performance compared to the
> > PAGE_SIZE buffer) and should not bring much overhead in terms of memory
> > consumption due to order 2 allocations.
> > 
> > Signed-off-by: Mikhail Zaslonko <zaslonko@xxxxxxxxxxxxx>
> 
> We may have to make these allocations under memory pressure in the IO context,
> order 2 allocations here is going to be not awesome.  If you really want it then
> you need to at least be able to fall back to single page if you fail to get the
> allocation.  Thanks,

The allocation is only for the workspace and it does not happen on the
IO path for each call. There's the pool and if

btrfs_get_workspace
  alloc_workspace

fails, then there's fallback path to wait for an existing workspace to
be free.

The order 2 allocation can put more pressure on the allocator though so
it's possible to have effects in some corner cases, but not in general.
I don't think the single page fallback code is needed.

And of course evaluation of the effects of the larger zlib buffer should
be done, it could improve compression but probably at the cost of cpu
time. Also decompression of blocks created on new code (4 pages) must
work on the old code (1 page).