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

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Message-ID: <20161017050424.GA4591@blaptop>
Date:   Mon, 17 Oct 2016 14:04:24 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Jens Axboe <axboe@...nel.dk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH 2/3] zram: support page-based parallel write

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.

Thanks.