linux-kernel - Re: [f2fs-dev] [PATCH] f2fs: fix double count on issued discard commands

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Message-ID: <20170912171157.GA2597@jaegeuk-macbookpro.roam.corp.google.com>
Date:   Tue, 12 Sep 2017 10:11:57 -0700
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Daeho Jeong <daeho.jeong@...sung.com>
Cc:     "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-f2fs-devel@...ts.sourceforge.net" 
        <linux-f2fs-devel@...ts.sourceforge.net>
Subject: Re: [f2fs-dev] [PATCH] f2fs: fix double count on issued discard
 commands

On 09/12, Daeho Jeong wrote:
> > Yeah, that's exactly like what I made a mistake before.
> > I should have mentioned that earlier. :)
> 
> Or I think the previous code which used "iter++" might be right.
> You might just want to check the fixed number of small discards, DISCARD_ISSUE_RATE,
> when issue_cond is "true".
> 
> Anyways, I have another question about this function.
> How about just issuing, not checking whether it is idle, the fixed number of small
> discards, DISCARD_ISSUE_RATE, when issue_cond is "true".
> Actually, the discard commands will be issued as "asynchronous" requests,
> which has a low priority in the I/O scheduler,
> so the performance degradation of other threads by doing this will not be much severe,
> but we can make the performance of the storage device better even if there is no idle.

I don't think I/O scheduler can efficiently prioritize discard commands and user
requests. The proper way that we can do would be waiting for idle time at this
moment.

Thanks,

> 
> I am just worried about the storage device I/O performance gets worse 
> under I/O intensive senario where there is no idle
> 
> Thanks,