How does sstate cache work in yocto?

Question

Can someone explain to me in more detail how the sstate cache works in yocto?

This explanation is far from clear: http://www.yoctoproject.org/docs/1.7/bitbake-user-manual/bitbake-user-manual.html#setscene

I do not understand when this situation arises:

NOTE: Preparing runqueue NOTE: Executing SetScene Tasks NOTE: Running setscene task 118 of 155 (virtual:native:/home/lulianhao/poky-build/edwin/poky/meta/recipes-devtools/pseudo/pseudo_git.bb:do_populate_sysroot_setscene) NOTE: Running setscene task 119 of 155 (/home/lulianhao/poky-build/edwin/poky/meta/recipes-devtools/quilt/quilt-native_0.48.bb:do_populate_sysroot_setscene) 

When he discovered artifacts or received candidates, he then checks for signatures. I want to know when setscene tasks are running.

Additional question: when does it look in the local sstate_cache folder and when in the mirror?

Answer 1

The Yocto project guide contains a section on the shared state cache:

http://www.yoctoproject.org/docs/latest/mega-manual/mega-manual.html#shared-state-cache

To answer your question, the sstate-cache folder is checked first, then the mirrors are checked if nothing is found locally.

Useful tip: "When we need to rebuild from scratch, we either remove build / tmp so that we can use sstate-cache to speed up the build, or we remove both build / tmp and sstate-cache so that no cache is reused during assembly. " ^one

_{¹ Salvador, Otavio and Diane Angolini. "6.2 Understanding the shared state cache." Embedded Linux development with the Yocto project}