Re: [PATCH 2/2] Migrate zone cache from RB-Tree to arrays of descriptors

[Hannes Reinecke]

On 08/22/2016 06:34 AM, Shaun Tancheff wrote:
> Currently the RB-Tree zone cache is fast and flexible. It does
> use a rather largish amount of ram. This model reduces the ram
> required from 120 bytes per zone to 16 bytes per zone with a
> moderate transformation of the blk_zone_lookup() api.
> 
> This model is predicated on the belief that most variations
> on zoned media will follow a pattern of using collections of same
> sized zones on a single device. Similar to the pattern of erase
> blocks on flash devices being progressivly larger 16K, 64K, ...
> 
> The goal is to be able to build a descriptor which is both memory
> efficient, performant, and flexible.
> 
> Signed-off-by: Shaun Tancheff <shaun.tancheff@xxxxxxxxxxx>
> ---
>  block/blk-core.c       |    2 +-
>  block/blk-sysfs.c      |   31 +-
>  block/blk-zoned.c      |  103 +++--
>  drivers/scsi/sd.c      |    5 +-
>  drivers/scsi/sd.h      |    4 +-
>  drivers/scsi/sd_zbc.c  | 1025 +++++++++++++++++++++++++++---------------------
>  include/linux/blkdev.h |   82 +++-
>  7 files changed, 716 insertions(+), 536 deletions(-)
> 
Have you measure the performance impact here?
The main idea behind using an RB-tree is that each single element will
fit in the CPU cache; using an array will prevent that.
So we will increase the number of cache flushes, and most likely a
performance penalty, too.
Hence I'd rather like to see a performance measurement here before going
down that road.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
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