Christian Ehrhardt wrote:
Johannes Weiner wrote:
[...]
It stays at ~85M with more writes which is approx 50% of my free 160M
memory.
Ok, so I am the idiot that got quoted on 'the active set is not too
big, so
buffer heads are not a problem when avoiding to scan it' in eternal
history.
But the threshold inactive/active ratio for skipping active file pages is
actually 1:1.
The easiest 'fix' is probably to change that ratio, 2:1 (or even 3:1?)
appears
to be a bit more natural anyway? Below is a patch that changes it to
2:1.
Christian, can you check if it fixes your regression?
I'll check it out.
from the numbers I have up to now I know that the good->bad transition
for my case is somewhere between 30M/60M e.g. first and second write.
The ratio 2:1 will eat max 53M of my ~160M that gets split up.
That means setting the ratio to 2:1 or whatever else might help or not,
but eventually there is just another setting of workload vs. memory
constraints that would still be affected. Still I guess 3:1 (and I'll
try that as well) should be enough to be a bit more towards the save side.
For "my case" 2:1 is not enough, 3:1 almost and 4:1 fixes the issue.
Still as I mentioned before I think any value carved in stone can and
will be bad to some use case - as 1:1 is for mine.
If we end up being unable to fix it internally by allowing the system to
"forget" and eventually free old unused buffers at least somewhen - then
we should neither implement it as 2:1 nor 3:1 nor whatsoever, but as
userspace configurable e.g. /proc/sys/vm/active_inactive_ratio.
I hope your suggestion below or an extension to it will allow the kernel
to free the buffers somewhen. Depending on how good/fast this solution
then will work we can still modify the ratio if needed.
Additionally, we can always scan active file pages but only deactivate
them
when the ratio is off and otherwise strip buffers of clean pages.
In think we need something that allows the system to forget its history
somewhen - be it 1:1 or x:1 - if the workload changes "long enough"(tm)
it should eventually throw all old things out.
Like I described before many systems have different usage patterns when
e.g. comparing day/night workload. So it is far from optimal if e.g. day
write loads eat so much cache and never give it back for nightly huge
reads tasks or something similar.
Would your suggestion achieve that already?
If not what kind change could?
[...]
--
Grüsse / regards, Christian Ehrhardt
IBM Linux Technology Center, System z Linux Performance
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxxx For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>