Re: MAP_SHARED bizarrely slow

[Andrew Morton]

James Cloos <cloos@xxxxxxxxxxx> wrote:
>
> >>>>> "David" == David Gibson <david@xxxxxxxxxxxxxxxxxxxxx> writes:
> 
> David> http://www.ozlabs.org/people/dgibson/maptest.tar.gz
> 
> David> On a number of machines I've tested - both ppc64 and x86 - the
> David> SHARED version is consistently and significantly (50-100%)
> David> slower than the PRIVATE version.
> 
> Just gave it a test on my laptop and server.  Both are p3.  The
> laptop is under heavier mem pressure; the server has just under
> a gig with most free/cache/buff.  Laptop is still running 2.6.7
> whereas the server is bk as of 2004-10-24.
> 
> Buth took about 11 seconds for the private and around 30 seconds
> for the shared tests.
> 

I get the exact opposite, on a P4:

vmm:/home/akpm/maptest> time ./mm-sharemmap 
./mm-sharemmap  10.81s user 0.05s system 100% cpu 10.855 total
vmm:/home/akpm/maptest> time ./mm-sharemmap
./mm-sharemmap  11.04s user 0.05s system 100% cpu 11.086 total
vmm:/home/akpm/maptest> time ./mm-privmmap 
./mm-privmmap  26.91s user 0.02s system 100% cpu 26.903 total
vmm:/home/akpm/maptest> time ./mm-privmmap
./mm-privmmap  26.89s user 0.02s system 100% cpu 26.894 total
vmm:/home/akpm/maptest> uname -a
Linux vmm 2.6.10-rc1-mm1 #14 SMP Tue Oct 26 23:23:23 PDT 2004 i686 i686 i386 GNU/Linux

It's all user time so I can think of no reason apart from physical page
allocation order causing additional TLB reloads in one case.  One is using
anonymous pages and the other is using shmem-backed pages, although I can't
think why that would make a difference.


Let's back out the no-buddy-bitmap patches:

vmm:/home/akpm/maptest> time ./mm-sharemmap 
./mm-sharemmap  12.01s user 0.06s system 99% cpu 12.087 total
vmm:/home/akpm/maptest> time ./mm-sharemmap
./mm-sharemmap  12.56s user 0.05s system 100% cpu 12.607 total
vmm:/home/akpm/maptest> time ./mm-privmmap 
./mm-privmmap  26.74s user 0.03s system 99% cpu 26.776 total
vmm:/home/akpm/maptest> time ./mm-privmmap
./mm-privmmap  26.66s user 0.02s system 100% cpu 26.674 total

much the same.

Backing out "[PATCH] tweak the buddy allocator for better I/O merging" from
June 24 makes no difference.

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