How to increase main memory via texmf-defaults.ini?

Hello,

I am running miktex 2.9 and currently compiling a .tex file involving a lot
pgf/tikz figures leads to shortage of memory(tried both xelatex
filename.tex or xelatex --main-memory=5000000 filename.tex),

+++++++++++++++++++++++++++++++++++++++++++++++++++++
Runaway definition?
->
! TeX capacity exceeded, sorry [main memory size=3000000].
\pgfplotslistpushbackglobal ...e \t@pgfplots@tokb
                                                  \the \t@pgfplots@toka }
l.141 ... ({5},{y},{sqrt(5)+(1+sqrt(5))*sqrt(y)});

--- HELP ---
No help available
+++++++++++++++++++++++++++++++++++++++++++++++++++++


I know in Texlive, I can increase the default value of 'main_memory= ' in
texmf.cnf to a greater number, then update the new setting with
fmtutil-sys. So I can succeed in compiling this file.

I saw a similar entry in texmf-defaults.ini under MIKTEXROOT/miktex/config/
and I increased the default number 3000000 to 5000000, but it doesn't help,
I don't know what's the corresponding command like fmtutil-sys in texlive
to refresh new setting. I tried 'refresh FNDB' and 'update formats' in
miktex options, they didn't help.

Any ideas are greatly appreciated. Thanks.

Best,
CHOU YU-CHIEH
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