Re: [PATCH 2/3] zram: support page-based parallel write

From: Minchan Kim
Date: Mon Oct 17 2016 - 01:05:02 EST

Hi Sergey,

On Fri, Oct 07, 2016 at 03:33:22PM +0900, Minchan Kim wrote:

< snip >

> > so the question is -- can we move this parallelization out of zram
> > and instead flush bdi in more than one kthread? how bad that would
> > be? can anyone else benefit from this?
> Isn't it blk-mq you mentioned? With blk-mq, I have some concerns.
> 1. read speed degradation
> 2. no work with rw_page
> 3. more memory footprint by bio/request queue allocation
> Having said, it's worth to look into it in detail more.
> I will have time to see that approach to know what I can do
> with that.

queue_mode=2 bs=4096 nr_devices=1 submit_queues=4 hw_queue_depth=128

Last week, I played with null_blk and blk-mq.c to get an idea how
blk-mq works and I realized it's not good for zram because it aims
to solve 1) dispatch queue bottleneck 2) cache-friendly IO completion
through IRQ so 3) avoids remote memory accesses.

For zram which is used for embedded as primary purpose, ones listed
abvoe are not a severe problem. Most imporant thing is there is no
model to support that a process queueing IO request on *a* CPU while
other CPUs issues the queued IO to driver.

Anyway, Although blk-mrq can support that model, it is blk-layer thing.
IOW, it's software stuff for fast IO delievry but what we need is
device parallelism of zram itself. So, although we follow blk-mq,
we still need multiple threads to compress in parallel which is most of
code I wrote in this patchset.

If I cannot get huge benefit(e.g., reduce a lot of zram-speicif code
to support such model) with blk-mq, I don't feel to switch to request
model at the cost of reasons I stated above.