How does getblocktemplate select transactions?

Question

I wrote a script that compares the transactions listed in the memory pool with the transactions included in getblocktemplate:

http://learnmeabitcoin.com/explorer/node/memorypool

(I've ordered the transactions by [current priority], and highlighted the ones included in getblocktemplate in blue.)

• How does getblocktemplate choose which transactions to include from the memory pool in a candidate block?

• Why aren't transactions being selected from highest to lowest priority?

Answer 1

The whole mempool won't fit in a block; getblocktemplate returns enough transactions for a block. Blocks are, by default, limited to 750kb by policy, but many miners increase the size to the block limit of 1mb (which can be done by command-line flag).

The specific piece of code that decides on the transactions that should be included in a block can be found here. It's reasonably well documented, and not terrible to read, I'd suggest taking a look through it to see where your script operates differently.