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From: Greg KH <>
To: yong w <>
Cc:,,,,,, David Hildenbrand <>,,,,,,,,,
Subject: Re: [RFC PATCH V3] zram:calculate available memory when zram is used
Date: Wed, 9 Jun 2021 16:59:58 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Wed, Jun 09, 2021 at 10:23:36PM +0800, yong w wrote:
> Greg KH <> 于2021年6月8日周二 下午5:29写道:
> >
> > On Mon, Jun 07, 2021 at 08:39:14AM -0700, wrote:
> > > From: wangyong <>
> > >
> > > When zram is used, available+Swap free memory is obviously bigger than we
> > > actually can use, because zram can compress memory by compression
> > > algorithm and zram compressed data will occupy memory too.
> > >
> > > So, we can count the compression ratio of zram in the kernel. The space
> > > will be saved by zram and other swap device are calculated as follows:
> > > zram[swapfree - swapfree * compress ratio] + swapdev[swapfree]
> > > We can evaluate the available memory of the whole system as:
> > > MemAvailable+zram[swapfree - swapfree * compress ratio]+swapdev[swapfree]
> >
> > Why is this needed to be exported by userspace?  Who is going to use
> > this information and why can't they just calculate it from the exported
> > information already?
> In embedded devices, it is necessary to assess how much memory is available.

Why is that any different from a server?  Or a laptop?  Or any other
system running Linux?  "embedded" isn't special here :)

> If the memory allocation is based on available+swapfree, it may cause oom and
> affect the normal use of the devices. And it is more accurate and safe
> to calculate
> the swap available memory through minimum compression ratio.
> Although mm_stat interface provides compressed information,but it is not easy to
> get the minimum compression ratio during swaping out. Kernel provides a common
> interface, which makes it easier to use and judge the state of system memory

If you are running up against this type of limit, how is a proc file
guess going to help much?  What are you going to do based on the result?
And as it's always going to be a guess, how reliable is it?

> > What tool requires this new information and what is it going to be used
> > for?
> It can be used in embedded devices to evaluate the memory condition
> and avoid causing oom; Also If we wants to know more accurate available
> memory information when zram is used.

Why not rely on the oom logic instead?  What is wrong with that as this
is always going to be just a guess.  You are never going to be able to
react fast enough to reading this value to be able to do anything better
than you could through the existing oom notifier/logic, right?

> > And why add a block driver's information to a core proc file?  Shouldn't
> > the information only be in the block driver's sysfs directory only?
> >
> > thanks,
> >
> > greg k-h
> I thought it would be better to put it there.

If no one needs it, why add it?  :)

> In the first patch, MemAvailable returned with swap available memory, and
> David recommended a separate interface.

A sysfs file makes more sense to me, and seems simpler.

But again, this is just a guess, trying to do real work based on it
feels really risky.


greg k-h

      reply	other threads:[~2021-06-09 15:00 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-07 15:39 [RFC PATCH V3] zram:calculate available memory when zram is used yongw.pur
2021-06-08  9:28 ` Greg KH
2021-06-09 14:58   ` yong w
2021-06-08  9:29 ` Greg KH
2021-06-09 14:23   ` yong w
2021-06-09 14:59     ` Greg KH [this message]

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