Re: swapping and the value of /proc/sys/vm/swappiness

[Marcelo Tosatti]

On Mon, Sep 06, 2004 at 09:03:04PM -0300, Marcelo Tosatti wrote:
> On Tue, Sep 07, 2004 at 09:34:20AM +1000, Con Kolivas wrote:
> > Andrew Morton writes:
> > 
> > >Con Kolivas <kernel@xxxxxxxxxxx> wrote:
> > >>
> > >>> A scan of the change logs for swappiness related changes shows nothing 
> > >>that > might explain these changes.  My question is:  "Is this change in 
> > >> behavior
> > >> > deliberate, or just a side effect of other changes that were made in 
> > >> the vm?" > and "What kind of swappiness behavior might I expect to find 
> > >> in future kernels?".
> > >>
> > >> The change was not deliberate but there have been some other people 
> > >> report significant changes in the swappiness behaviour as well (see 
> > >> archives). It has usually been of the increased swapping variety lately. 
> > >> It has been annoying enough to the bleeding edge desktop users for a 
> > >> swag of out-of-tree hacks to start appearing (like mine).
> > >
> > >All of which is largely wasted effort.  It would be much more useful to get
> > >down and identify which patch actually caused the behavioural change.
> > 
> > I don't disagree. Is there anyone who has the time and is willing to do the 
> > regression testing? This is a general appeal to the mailing list.
> 
> Hi kernel fellows,
> 
> I volunteer. I'll try something tomorrow to compare swappiness of older kernels like  
> 2.6.5 and 2.6.6, which were fine on SGI's Altix tests, up to current newer kernels 
> (on small memory boxes of course).
> 
> Someone needs to write a vmstat-like tool to parse /proc/vmstat. 
> The statistics in there allows us to watch the behaviour of VM
> page reclaim code.
> 
> Con, if you could compile a list of reports we would be very grateful.

Spent some time doing a few tests.

A contained test (512MB box, allocate 450MB of memory, touch and sleep + 
huge dd) show's that 2.6.6 and 2.6.7 are equivalent (swapout 120-150M when the "dd" 
starts writing out data).

2.6.9-rc1 swaps out 350M on the same test. Doenst seem nice, I'll try to isolate 
what causes this tomorrow (this is probably what is hitting desktop users which
is fixed by Con's patches). Thats problem #1. Something needs a little tweaking 
I think. 

Ray, I see the additional swapouts increase the dd performance for your particular testcase:

on 2.6.6:
        Total I/O   Avg Swap   min    max     pg cache    min    max
       ----------- --------- ------- ------  --------- ------- -------
   0   242.47 MB/s      0 MB (     0,     0)   3195 MB (  3138,  3266)
  20   256.06 MB/s      0 MB (     0,     0)   3170 MB (  3074,  3234)
  40   267.29 MB/s      0 MB (     0,     0)   3189 MB (  3137,  3234)
  60   289.43 MB/s    666 MB (    72,  1680)   3847 MB (  3296,  4817)		<---------- 

So for this one testcase it is being beneficial. 

However, decreasing the "swappiness" value does not seem to make much of a difference:

Kernel Version 2.6.8.1-mm4:
        Total I/O   Avg Swap   min    max     pg cache    min    max
       ----------- --------- ------- ------  --------- ------- -------
   0   287.28 MB/s    710 MB (    46,  3060)   4082 MB (  3426,  6308)
  20   288.05 MB/s    508 MB (    94,  1417)   3848 MB (  3442,  4739)
  40   287.03 MB/s    588 MB (   199,  1251)   3909 MB (  3570,  4515)
  60   290.08 MB/s    640 MB (   210,  1190)   3976 MB (  3538,  4531)
  80   287.73 MB/s    693 MB (   316,  1195)   4049 MB (  3713,  4545)
 100   166.17 MB/s  26001 MB ( 26001, 26002)  28798 MB ( 28740, 28852)
                                                                                                                                                                                   
Kernel Version 2.6.9-rc1-mm3:
        Total I/O   Avg Swap   min    max     pg cache    min    max
       ----------- --------- ------- ------  --------- ------- -------
   0   274.80 MB/s  10511 MB (  5644, 14492)  13293 MB (  8596, 17156)
  20   267.02 MB/s  12624 MB (  5578, 16287)  15298 MB (  8468, 18889)
  40   267.66 MB/s  13541 MB (  6619, 17461)  16199 MB (  9393, 20044)
  60   233.73 MB/s  18094 MB ( 16550, 19676)  20629 MB ( 19103, 22192)
  80   213.64 MB/s  20950 MB ( 15844, 22977)  23450 MB ( 18496, 25440)
 100   164.58 MB/s  26004 MB ( 26004, 26004)  28410 MB ( 28327, 28455)

And that is a problem #2 - swappinness not being honoured. Guys, 
any ideas on the reason for that?

Andrew, dirty_ratio and dirty_background_ratio (as low as 5% each) did not significantly 
affect the amount of swapped out data, only a small effect on _how soon_ anonymous 
memory was swapped out.

And finally, Ray, the difference you see between 2.6.6 and 2.6.7 can be explained, 
as noted by others in this thread, to vmscan.c changes (page replacement/scanning policy
changes were made).

Will continue with more tests tomorrow.
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