freebsd stable - Jun 2020 - swap space issues

> On 26 Jun 2020, at 11:23, Peter Jeremy <peter at rulingia.com> wrote:
> 
> On 2020-Jun-25 11:30:31 -0700, Donald Wilde <dwilde1 at gmail.com>
wrote:
>> Here's 'pstat -s' on the i3 (which registers as cpu
HAMMER):
>> 
>> Device          1K-blocks     Used    Avail Capacity
>> /dev/ada0s1b     33554432        0 33554432     0%
>> /dev/ada0s1d     33554432        0 33554432     0%
>> Total            67108864        0 67108864     0%
> 
> I strongly suggest you don't have more than one swap device on spinning
> rust - the VM system will stripe I/O across the available devices and
> that will give particularly poor results when it has to seek between the
> partitions.
If you configure a ZFS mirror in bsdinstall you get a swap partition per drive
by default.
> Also, you can't actually use 64GB swap with 4GB RAM.  If you look back
> through your boot messages, I expect you'll find messages like:
> warning: total configured swap (524288 pages) exceeds maximum recommended
amount (498848 pages).
> warning: increase kern.maxswzone or reduce amount of swap.
> or maybe:
> WARNING: reducing swap size to maximum of xxxxMB per unit
> 
> The absolute limit on swap space is vm.swap_maxpages pages but the
realistic
> limit is about half that.  By default the realistic limit is about 4?RAM
(on
> 64-bit architectures), but this can be adjusted via kern.maxswzone (which
> defines the #bytes of RAM to allocate to swzone structures - the actual
> space allocated is vm.swzone).
> 
> As a further piece of arcana, vm.pageout_oom_seq is a count that controls
> the number of passes before the pageout daemon gives up and starts killing
> processes when it can't free up enough RAM.  "out of swap
space" messages
> generally mean that this number is too low, rather than there being a
> shortage of swap - particularly if your swap device is rather slow.
> 
> --
> Peter Jeremy

--
Bob Bishop       t: +44 (0)118 940 1243
rb at gid.co.uk     m: +44 (0)783 626 4518





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Van: Bob Bishop <rb at gid.co.uk>
Datum: vrijdag, 26 juni 2020 17:18
Aan: Peter Jeremy <peter at rulingia.com>
CC: Donald Wilde <dwilde1 at gmail.com>, freebsd-stable <freebsd-stable
at freebsd.org>
Onderwerp: Re: swap space issues
> 
> 
> 
> > On 26 Jun 2020, at 11:23, Peter Jeremy <peter at rulingia.com>
wrote:
> >
> > On 2020-Jun-25 11:30:31 -0700, Donald Wilde <dwilde1 at
gmail.com> wrote:
> >> Here's 'pstat -s' on the i3 (which registers as cpu
HAMMER):
> >>
> >> Device          1K-blocks     Used    Avail Capacity
> >> /dev/ada0s1b     33554432        0 33554432     0%
> >> /dev/ada0s1d     33554432        0 33554432     0%
> >> Total            67108864        0 67108864     0%
> >
> > I strongly suggest you don't have more than one swap device on
spinning
> > rust - the VM system will stripe I/O across the available devices and
> > that will give particularly poor results when it has to seek between
the
> > partitions.
>  
> If you configure a ZFS mirror in bsdinstall you get a swap partition per
drive by default.
If you are running on multiple disks (a mirror) it can provide extra speed. The
example above is on the same disk. On one disk multiple swap partitions will
only spread the data non-optimal for the heads of the disk.


 > 
> > Also, you can't actually use 64GB swap with 4GB RAM.  If you look
back
> > through your boot messages, I expect you'll find messages like:
> > warning: total configured swap (524288 pages) exceeds maximum
recommended amount (498848 pages).
> > warning: increase kern.maxswzone or reduce amount of swap.
> > or maybe:
> > WARNING: reducing swap size to maximum of xxxxMB per unit
> >
> > The absolute limit on swap space is vm.swap_maxpages pages but the
realistic
> > limit is about half that.  By default the realistic limit is about
4?RAM (on
> > 64-bit architectures), but this can be adjusted via kern.maxswzone
(which
> > defines the #bytes of RAM to allocate to swzone structures - the
actual
> > space allocated is vm.swzone).
> >
> > As a further piece of arcana, vm.pageout_oom_seq is a count that
controls
> > the number of passes before the pageout daemon gives up and starts
killing
> > processes when it can't free up enough RAM.  "out of swap
space" messages
> > generally mean that this number is too low, rather than there being a
> > shortage of swap - particularly if your swap device is rather slow.
> >
> > --
> > Peter Jeremy
> 
> 
> --
> Bob Bishop       t: +44 (0)118 940 1243
> rb at gid.co.uk     m: +44 (0)783 626 4518
> 
> 
> 
> 
>  
> 
> 
>