Subject: port-i386/37193: x86 pmap concurrency strategy could use improvement
To: None <,,>
From: None <>
List: netbsd-bugs
Date: 10/24/2007 08:10:01
>Number:         37193
>Category:       port-i386
>Synopsis:       x86 pmap concurrency strategy could use improvement
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    port-i386-maintainer
>State:          open
>Class:          change-request
>Submitter-Id:   net
>Arrival-Date:   Wed Oct 24 08:10:01 +0000 2007
>Originator:     Andrew Doran
>Release:        4.99.34
The NetBSD Project
This applies to the vmlocking branch but the same (unused) strategy
is in HEAD.

- It should be possible to use atomics to adjust the pmap reference
  count, instead of adjusting the count under lock. The uvm_object
  is passed into MI code in one or two places. Need to check any
  reference count changes made by those calls.

- The per-CPU pv cache generates contention and adds an extra 4 bytes
  to pv_head. Since the allocations are now done without holding pmap 
  locks, it should be possible to change it to use pool_cache instead.

- pmap_main_lock is 'cache hot' due to it being taken on nearly every
  pmap operation; it would be nice to get rid of it.

- pmap_test_attrs/pmap_clear_attrs acquire too many locks. The global
  pmap_main_lock is write locked by these routines, so it causes
  contention on pmap_main_lock and back-pressure on other locks like
  uvm_pageqlock. Also, they scroll through all the pmaps that have the
  target page mapped and lock/unlock them. pmap_page_remove has a
  similar problem but it does not appear to be called much.

- The splay tree should probably be replaced by a red-black tree.
  Lookup operations are modifying on a splay tree and that's likely
  cause false sharing of cache lines between CPUs.
Code inspection / testing.
See above.. For pmap_test_attrs/pmap_clear_attrs:

1. Lock the current pmap in order to make use of its APTE space.
   Provide a per-CPU APTE space and disable preemption to use it.
2. Lock the pv_head. It will prevent the referenced pmaps from
   disappearing while we operate on them.
3. Scroll through each pmap, mapping whatever is necessary into
   the APTE space, and do the operation.