Add instructions for multi-processor gitian builds

Author: coblee

doc/gitian-building.md

@@ -76,7 +76,11 @@ In the VirtualBox GUI click "New" and choose the following parameters in the wiz
 
 After creating the VM, we need to configure it.
 
-- Click the `Settings` button, then go to the `Network` tab. Adapter 1 should be attached to `NAT`.
+- Click the `Settings` button, then go to `System` tab and `Processor` sub-tab. Increase the number of processors to the number of cores on your machine if you want builds to be faster.
+
+![](gitian-building/system_settings.png)
+
+- Go to the `Network` tab. Adapter 1 should be attached to `NAT`.
 
 ![](gitian-building/network_settings.png)
 

doc/gitian-building/system_settings.png

@@ -111,16 +111,16 @@ The gbuild invocations below <b>DO NOT DO THIS</b> by default.
 ### Build and sign Bitcoin Core for Linux, Windows, and OS X:
 
     pushd ./gitian-builder
-    ./bin/gbuild --memory 3000 --commit bitcoin=v${VERSION} ../bitcoin/contrib/gitian-descriptors/gitian-linux.yml
+    ./bin/gbuild --num-make 2 --memory 3000 --commit bitcoin=v${VERSION} ../bitcoin/contrib/gitian-descriptors/gitian-linux.yml
     ./bin/gsign --signer $SIGNER --release ${VERSION}-linux --destination ../gitian.sigs/ ../bitcoin/contrib/gitian-descriptors/gitian-linux.yml
     mv build/out/bitcoin-*.tar.gz build/out/src/bitcoin-*.tar.gz ../
 
-    ./bin/gbuild --memory 3000 --commit bitcoin=v${VERSION} ../bitcoin/contrib/gitian-descriptors/gitian-win.yml
+    ./bin/gbuild --num-make 2 --memory 3000 --commit bitcoin=v${VERSION} ../bitcoin/contrib/gitian-descriptors/gitian-win.yml
     ./bin/gsign --signer $SIGNER --release ${VERSION}-win-unsigned --destination ../gitian.sigs/ ../bitcoin/contrib/gitian-descriptors/gitian-win.yml
     mv build/out/bitcoin-*-win-unsigned.tar.gz inputs/bitcoin-win-unsigned.tar.gz
     mv build/out/bitcoin-*.zip build/out/bitcoin-*.exe ../
 
-    ./bin/gbuild --memory 3000 --commit bitcoin=v${VERSION} ../bitcoin/contrib/gitian-descriptors/gitian-osx.yml
+    ./bin/gbuild --num-make 2 --memory 3000 --commit bitcoin=v${VERSION} ../bitcoin/contrib/gitian-descriptors/gitian-osx.yml
     ./bin/gsign --signer $SIGNER --release ${VERSION}-osx-unsigned --destination ../gitian.sigs/ ../bitcoin/contrib/gitian-descriptors/gitian-osx.yml
     mv build/out/bitcoin-*-osx-unsigned.tar.gz inputs/bitcoin-osx-unsigned.tar.gz
     mv build/out/bitcoin-*.tar.gz build/out/bitcoin-*.dmg ../