linux-kernel - Re: [PATCH v1 2/6] fs: use on-stack-bio if backing device has BDI_CAP

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Message-ID: <058b4ae5-c6e9-ff32-6440-fb1e1b85b6fd@kernel.dk>
Date:   Mon, 14 Aug 2017 10:17:09 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        "karam . lee" <karam.lee@....com>, seungho1.park@....com,
        Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
        Vishal Verma <vishal.l.verma@...el.com>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        kernel-team <kernel-team@....com>
Subject: Re: [PATCH v1 2/6] fs: use on-stack-bio if backing device has
 BDI_CAP_SYNC capability

On 08/14/2017 09:38 AM, Jens Axboe wrote:
> On 08/14/2017 09:31 AM, Minchan Kim wrote:
>>> Secondly, generally you don't have slow devices and fast devices
>>> intermingled when running workloads. That's the rare case.
>>
>> Not true. zRam is really popular swap for embedded devices where
>> one of low cost product has a really poor slow nand compared to
>> lz4/lzo [de]comression.
> 
> I guess that's true for some cases. But as I said earlier, the recycling
> really doesn't care about this at all. They can happily coexist, and not
> step on each others toes.

Dusted it off, result is here against -rc5:

http://git.kernel.dk/cgit/linux-block/log/?h=cpu-alloc-cache

I'd like to split the amount of units we cache and the amount of units
we free, right now they are both CPU_ALLOC_CACHE_SIZE. This means that
once we hit that count, we free all of the, and then store the one we
were asked to free. That always keeps 1 local, but maybe it'd make more
sense to cache just free CPU_ALLOC_CACHE_SIZE/2 (or something like that)
so that we retain more than 1 per cpu in case and app preempts when
sleeping for IO and the new task on that CPU then issues IO as well.
Probably minor.

Ran a quick test on nullb0 with 32 sync readers. The test was O_DIRECT
on the block device, so I disabled the __blkdev_direct_IO_simple()
bypass. With the above branch, we get ~18.0M IOPS, and without we get
~14M IOPS. Both ran with iostats disabled, to avoid any interference
from that.

-- 
Jens Axboe