[Ethereum] How Ethereum estimate the gas for running a contract

Yes, a transaction's gas limit can have an effect on the time it takes to get mined into a block.

Larger gas limits can be more enticing to miners since their potential fee is gas limit * gas price. However, this is a potential fee, since unused gas is refunded. Thus, it is possible that transactions with very high gas limits, can be prioritized lower by miners and take longer to get included in a block.

Consider the case where there's 11 transactions:

• 1 transaction T has a gas limit equal to the block gas limit.
• 10 other transactions have a gas limit that's 1/10th of the block gas limit
• All 11 transactions have equal gas price

If you're the miner, which of the transactions would you include first?

By choosing T first, you have reached the block gas limit and cannot include any other transactions in the block. If T actually only used 1/10th of the gas, you lost the opportunity to include other transactions and collect their fees. In this limited example, there is more likelihood of getting more fees by including the 10 transactions first.

That's some theory and the default behavior of clients like Geth and Parity would require examining their code. But miners can choose to modify the default behavior.

Gas Limit (maximum number of gas units required for the transaction to execute):

For this contract, I believe that you won't need more than 1 million gas units. You can put 2 million, I guess there's not much harm in that except for making your transaction take longer to execute. But you cannot put more than the block limit, which is typically around 8 million, depending on what network you're deploying to (mainnet, ropsten, your own private network, etc).

Value (amount of wei-ether passed within the transaction to the contract):

Since you don't have a payable fallback function in the contract, you cannot put anything other than 0, because the transaction will fail otherwise (in fact, that's probably the reason for your error).