  Let $(\E,\F)$ be a symmetric non-local Dirichlet from with unbounded
coefficient on $L^2(\R^d;\d x)$ defined by $$\E(f,g)=\iint_{\R^d\times \R^d}
(f(y)-f(x))(g(x)-g(y)){W(x,y)}\, J(x,\d y)\,\d x, \quad f,g\in \F,$$ where
$J(x,\d y)$ is regarded as the jumping kernel for a pure-jump symmetric
L\'evy-type process with bounded coefficients, and $W(x,y)$ is seen as a
weighted (unbounded) function. We establish sharp criteria for compactness and
non-compactness of the associated Markovian semigroup $(P_t)_{t\ge0}$ on
$L^2(\R^d;\d x)$. In particular, we prove that if $J(x,\d
y)=|x-y|^{-d-\alpha}\,\d y$ with $\alpha\in (0,2)$, and $$W(x,y)= \begin{cases}
(1+|x|)^p+(1+|y|)^p, \ & |x-y|< 1 \\ (1+|x|)^q+(1+|y|)^q, \ & |x-y|\geq 1
\end{cases}$$ with $p\in [0,\infty)$ and $q\in [0,\alpha)$, then
$(P_t)_{t\ge0}$ is compact, if and only if $p>2$. This indicates that the
compactness of $(\E,\F)$ heavily depends on the growth of the weighted function
$W(x,y)$ only for $|x-y|<1$. Our approach is based on establishing the
essential super Poincar\'e inequality for $(\E,\F)$. Our general results work
  even if the jumping kernel $J(x,\d y)$ is degenerate or is singular
  with respect to the Lebesgue measure.
