K 10
svn:author
V 6
andrew
K 8
svn:date
V 27
2016-09-16T12:39:21.491248Z
K 7
svn:log
V 1174
MFC 305545:
Only call cpu_icache_sync_range when inserting an executable page. If the
page is non-executable the contents of the i-cache are unimportant so this
call is just adding unneeded overhead when inserting pages.

While doing research using gem5 with an O3 pipeline and 1k/32k/1M iTLB/L1
iCache/L2 Bjoern Zeeb (bz@) observed a fairly high rate of calls into
arm64_icache_sync_range() from pmap_enter() along with a high number of
instruction fetches and iTLB/iCache hits.

Limiting the calls to arm64_icache_sync_range() to only executable pages,
we observe the iTLB and iCache Hit going down by about 43%. These numbers
are quite misleading when looked at alone as at the same time instructions
retired were reduced by 19.2% and instruction fetches were reduced by 38.8%.
Overall this reduced the runtime of the test program by 22.4%.

On Juno hardware, in steady-state, running the same test, using the cycle
count to determine runtime, we do see a reduction of up to 28.9% in runtime.

While these numbers certainly depend on the program executed, we expect an
overall performance improvement.

Obtained from:	ABT Systems Ltd
Sponsored by:	The FreeBSD Foundation

END