Mempool Eviction

Mempool eviction is what a Bitcoin node does when its memory pool fills up: to stay within its configured size cap it discards the lowest-feerate transactions to make room for higher-paying ones. The default cap is set by -maxmempool (300 MB in Bitcoin Core), and once the pool reaches it, the node trims from the bottom of the fee market rather than refusing all new transactions outright.

How eviction works

Transactions are ranked by their feerate including descendants, so a low-fee parent kept alive by a high-fee child is scored on the package. When space is needed, the node evicts the lowest-scoring transaction (and its descendants, since a child cannot remain without its parent). Crucially, eviction also raises the node's effective minimum relay feerate to just above the feerate of the last transaction evicted, so during congestion a node will reject anything cheaper than what it just dropped — which is why low-fee transactions can vanish from the network during a fee spike and need re-broadcasting later.

Why it matters

Eviction is the mechanism that makes the mempool a true fee market under load: block space is scarce, and when demand exceeds it, the cheapest transactions are simply forgotten until fees fall. For anyone sending Bitcoin, the practical lesson is to either pay a competitive feerate or be prepared to fee-bump or rebroadcast a transaction that gets evicted. It is also a denial-of-service safeguard, bounding how much memory an adversary can force a node to consume.

Eviction is governed by mempool policy and the ancestor/descendant limits that bound transaction chains; avoiding it is a matter of accurate fee estimation.