用 tikz 绘制的椭圆边缘锯齿状，不够平滑

Question

我用 tikz 绘制的椭圆不平滑，边缘参差不齐，如下所示

在序言中使用了以下内容

    \tikzstyle{eff} = [ellipse,  minimum height=2em,  draw=black, fill=pink!20]

最后

\documentclass[10pt,a4paper]{article}
\usepackage{printlen}
\usepackage{layouts}
\usepackage{titlesec}
\usepackage{multirow, makecell, tabularx, booktabs}
\usepackage{babel}
\usepackage{setspace}
\usepackage{tgschola}
\usepackage[
  height=8.5in,      % height of the text block
  width=6.5in,       % width of the text block
  top=78pt,        % distance of the text block from the top of the page
  headheight=60pt, % height for the header block
  headsep=12pt,    % distance from the header block to the text block
  heightrounded,   % ensure an integer number of lines
  %showframe,       % show the main blocks
  verbose,         % show the values of the parameters in the log file
]{geometry}
\usepackage{enumitem}
\usepackage[table]{xcolor}                      %for color of table
\usepackage{booktabs}                        %for toprule midrule bottomrule in tables
\usepackage{multirow} % Required for multirows
\usepackage{longtable}
\usepackage{rotating}
\usepackage{lscape}
\usepackage[utf8]{inputenc}
%\usepackage{amsmath}
%\usepackage{amsfonts, amsmath, amssymb}                    %maths
%\usepackage{amssymb}
\usepackage{fancyhdr}
\usepackage[none]{hyphenat}                             %no hyphenation
\usepackage{graphicx}
\usepackage{float}
\usepackage{lastpage}                                     %toget last page number
 \usepackage{indentfirst}
 \usepackage{smartdiagram}
% \usepackage{dtklogos}
\usepackage{tikz}
\usetikzlibrary{shapes, arrows, matrix, calc}
\usetikzlibrary{decorations.markings}
\usepackage{verbatim}
\tikzstyle{process} = [rectangle, minimum width=3cm, minimum height=1cm, text centered, text width=2.5cm, draw=black, fill=blue!20]
\tikzstyle{eff} = [ellipse,  minimum height=2em,  draw=black, fill=pink!20]
\tikzstyle{arrow} = [thick,  ->, >=stealth]



\begin{tikzpicture}[node distance=2cm]

\node (pro1)[process]{Non\\ Conformance Identified};
\node (pro2a)[process, below of =pro1, xshift=-5cm]{Service};
\node (pro2b)[process, below of =pro1]{Process};
\node (pro2c)[process, below of =pro1, xshift=5cm]{System};
\node (pro3)[process, below of =pro2b]{Investigation of Cause};
\node (pro4)[process, below of =pro3]{Corrective Action Plan};
\node (pro5)[process, below of =pro4]{Implementation of Action Plan};
\node (pro4a)[process, right of =pro4, xshift=3cm, yshift=-1cm]{Review of Plan, \\Action \&\ Effectiveness in Management Review Meeting};
\node (eff)[eff, below of = pro4a, yshift = -.5cm]{Check Effectiveness};
\node (pro4b)[process, below of = pro4a, yshift = -2cm]{Is it OK?};
\node (pro4c)[process, below of = pro4b]{Close};

\draw [arrow](pro1)--(pro2b);
\draw [arrow](pro1)--(pro2a);
\draw [arrow](pro1)--(pro2c);
\draw [arrow](pro2b)--(pro3);
\draw [arrow](pro3)--(pro4);
\draw [arrow](pro4)--(pro5);
% \draw [arrow](pro4)--(pro4a);
% \draw [arrow](pro5)--(pro4a);
\draw [arrow] (pro2a) |- ($(pro2b)!.5!(pro3)$);
\draw [arrow] (pro2c) |- ($(pro2b)!.5!(pro3)$);
\draw [arrow] (eff) -- (pro4b);
\draw [arrow] (pro4b) -- (pro4c);
\draw [arrow](pro5)|-(eff);
\draw[arrow](1.5,-6)--(3.5,-6);
\draw[arrow](1.5,-8)--(3.5,-8);
\draw[arrow](3.5,-11)--(-5,-11)|-(-1.5,-4);

\node [align=left] at (2.6,-11.2) {Not OK};
\node [align=left] at (5.5,-12) {OK};
\end{tikzpicture}

如何让它顺利