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

[Shaun Tancheff]

On Mon, Aug 22, 2016 at 2:11 AM, Hannes Reinecke <hare@xxxxxxx> wrote:
> 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?

As far as actual hardware (HostAware) I am seeing the same
I/O performance. I suspect its just that below 100k iops the
zone cache just isn't a bottleneck.

> 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.

I think it will have to be a simulated benchmark, if that's okay.

Of course I'm open to suggestions if there is something you have in mind.
-- 
Regards,
Shaun Tancheff