On 15/09/2017 1:23 PM, Mel Gorman wrote:
On Thu, Sep 14, 2017 at 07:49:31PM +0300, Tariq Toukan wrote:
Insights: Major degradation between #1 and #2, not getting any
close to linerate! Degradation is fixed between #2 and #3. This is
because page allocator cannot stand the higher allocation rate. In
#2, we also see that the addition of rings (cores) reduces BW (!!),
as result of increasing congestion over shared resources.
Unfortunately, no surprises there.
Congestion in this case is very clear. When monitored in perf top:
85.58% [kernel] [k] queued_spin_lock_slowpath
While it's not proven, the most likely candidate is the zone lock
and that should be confirmed using a call-graph profile. If so, then
the suggestion to tune to the size of the per-cpu allocator would
mitigate the problem.
Indeed, I tuned the per-cpu allocator and bottleneck is released.
I think that page allocator issues should be discussed separately:
1) Rate: Increase the allocation rate on a single core. 2)
Scalability: Reduce congestion and sync overhead between cores.
This is clearly the current bottleneck in the network stack receive
flow.
I know about some efforts that were made in the past two years. For
example the ones from Jesper et al.: - Page-pool (not accepted
AFAIK).
Indeed not and it would also need driver conversion.
- Page-allocation bulking.
Prototypes exist but it's pointless without the pool or driver
conversion so it's in the back burner for the moment.
As I already mentioned in another reply (to Jesper), this would
perfectly fit with our Striding RQ feature, as we have large descriptors
that serve several packets, requiring the allocation of several pages at
once. I'd gladly move to using the bulking API.
- Optimize order-0 allocations in Per-Cpu-Pages.
This had a prototype that was reverted as it must be able to cope
with both irq and noirq contexts.
Yeah, I remember that I tested and reported the issue.
Unfortunately I never found the time to
revisit it but a split there to handle both would mitigate the
problem. Probably not enough to actually reach line speed though so
tuning of the per-cpu allocator sizes would still be needed. I don't
know when I'll get the chance to revisit it. I'm travelling all next
week and am mostly occupied with other work at the moment that is
consuming all my concentration.
I am not an mm expert, but wanted to raise the issue again, to
combine the efforts and hear from you guys about status and
possible directions.
The recent effort to reduce overhead from stats will help mitigate
the problem.
I should get more familiar with these stats, check how costly they are,
and whether they can be turned off in Kconfig.
Finishing the page pool, the bulk allocator and converting drivers
would be the most likely successful path forward but it's currently
stalled as everyone that was previously involved is too busy.
I think we should consider changing the default allocation of PCP
fraction as well, or implement some smart dynamic heuristic.
This turned on to have significant effect over networking performance.
Many thanks Mel!
Regards,
Tariq
