memoryclerk consumes a lot of memory

Hi  guys,

Could you explain me why memoryclerk_sqloptimizer consumes a lot of memory and how to solve it (release memory)?

Please have a look reports below.

1. Before

2. After 5 days

So we can  see that sqloptimizer became to consume about 1.9Gb instead of 1Gb  initially (in 1 day after server restart this value was just about 700Mb).

After some days this behavior will lead to server  to be almost unavailable for application.

Please suggest what can be a cause and how I can change this situation.

Max memory 8Gb (server 32Gb).

Microsoft SQL Server 2008 R2 (RTM) - 10.50.1600.1 (X64)