Re: Posix Shared Mem patch

Hi All,

In a *very* quick patch I tested using huge pages/MAP_HUGETLB for the mmap'ed 
memory.
That gives around 9.5% performance benefit in a read-only pgbench run (-n -S -
j 64 -c 64 -T 10 -M prepared, scale 200, 6GB s_b, 8 cores, 24GB mem).

It also saves a bunch of memory per process due to the smaller page table 
(shared_buffers 6GB):
cat /proc/$pid_of_pg_backend/status |grep VmPTE
VmPTE:        6252 kB
vs
VmPTE:          60 kB

Additionally it has the advantage that top/ps/... output under linux now looks 
like:                                                                         PID USER      PR  NI  VIRT  RES  SHR S
%CPU%MEM    TIME+  COMMAND 
 
10603 andres    20   0 6381m 4924 1952 R    21  0.0   0:28.04 postgres  

i.e. RES now actually shows something usable... Which is rather nice imo.

I don't have the time atm into making this something useable, maybe somebody 
else want to pick it up? Looks pretty worthwile investing some time.

Because of the required setup we sure cannot make this the default but...

Greetings,

Andres
-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
Services