why does the bibtex generate no bibitem in the beamer?

Question

I use the bibtex to generate the bibliography in my beamer, but I get warning about empty thebibliography environment.

My spmv.bib file:

    @thesis{im_optimizing_2000,
    title = {Optimizing the Performance of Sparse Matrix-Vector Multiplicatio},
    institution = {{EECS} Department, University of California, Berkeley},
    type = {phdthesis},
    author = {Im, Eun-jin},
    date = {2000-06},
}

@thesis{vuduc_automatic_2003,
    title = {Automatic Performance Tuning of Sparse Matrix Kernels},
    institution = {University of California, Berkeley},
    type = {phdthesis},
    author = {Vuduc, Richard Wilson},
    date = {2003},
    note = {{AAI}3121741},
}

My beamer file:

\documentclass{beamer}

\mode<presentation>
{
  \usetheme{CambridgeUS}
  \usecolortheme{dolphin}
  \usecolortheme{rose}
  \setbeamerfont*{title}{shape=\itshape, family=\rmfamily}
  \setbeamercovered{transparent}
}

\usepackage[english]{babel}
\usepackage[latin1]{inputenc}
\usepackage{times}
\usepackage[T1]{fontenc}


\begin{document}
\begin{frame}
  \begin{block}{References}
    \bibliographystyle{plain}
    \bibliography{spmv}
  \end{block}
\end{frame} 

\end{document}

The output of blg file:

This is BibTeX, Version 0.99dThe top-level auxiliary file: test.aux
The style file: plain.bst
I found no \citation commands---while reading file test.aux
Database file #1: spmv.bib

(There was 1 error message The output of bbl file:

\begin{thebibliography}{}

\end{thebibliography}

The output of beamer: