Re: Align large shared memory allocations

Tom Lane wrote:

>Manfred Spraul <manfred(at)colorfullife(dot)com> writes:
>
>
>>Attached is a patch that aligns large shared memory allocations beyond
>>MAXIMUM_ALIGNOF. The reason for this is that Intel's cpus have a fast
>>path for bulk memory copies that only works with aligned addresses.
>>
>>
>
>This patch is missing a demonstration that it's actually worth anything.
>What kind of performance gain do you get?
>
>
7.4cvs on a 1.13 GHz Intel Celeron mobile, 384 MB RAM, "Severn" RedHat
Linux 2.4 beta, postmaster -N 30 -B 64, data directory on ramdisk,
pgbench -c 10 -s 11 -t 1000:
Without the patch: 124 tps
with the patch: 130 tps.

I've reduced the buffer setting to 64 because without that, a too large
part of the database was cached by postgres. I expect that with all
Intel Pentium III chips, it will be worth 10-20% less system time. I had
around 30% system time after reducing the number of buffers, thus the
~5% performance improvement.

>We don't really have arch-dependent header files. What I'd be inclined
>to do is "#define ALIGNOF_BUFFER 32" in pg_config_manual.h, then
>#define BUFFERALIGN(LEN) to parallel the other TYPEALIGN macros in c.h,
>and finally use that in the ShmemAlloc code.
>
>
Ok, new patch attached.

--
Manfred