"out of memory" error in building light

hi. i have this problem in some of my projects. each time my project get a little big , i will face this error, and never could solve it. now this time, i just use a 2x3 landscape, with sky sphere and direction light. i didn’t do any specific thing. just cover the landscape with grasses that imported from “open world demo collection”. the number of grasses are 253k. please help me, i need to know where is my problem, and solve it.
my PC hardware :

AMD FX-8350 Eight-core 
HD Radeon 5870 1 GB DDR5
8 GB Ram DDR3-1600
and my windows is seven 64-bit

if you need any file just tell me.

Hi Khokialism,

As the error suggests you’re running out of memory when building lighting. Large/Medium worlds that have a lot of foliage and objects that need to be calculated will use a lot of RAM and CPU to process the lighting that needs to be baked.

You can see the suggestions here in our Swarm Agent troubleshooting and Tips guide that may help resolve your issues: A new, community-hosted Unreal Engine Wiki - Announcements - Epic Developer Community Forums

Thank you!