Currently the mempool isn't persisted across restarts. The historical argument against doing this is that it would make transactions immortal. Expiration now addresses that issue.

Without preserving the mempool node behavior changes drastically across restarts which complicates production use (e.g. mining overly small/low fee blocks) as well as development testing. The lack of retention also causes the loss of txn priority modifications made with prioritisetransaction which makes the API less reliable.

while (!parentHashes.empty()) {
        txiter stageit = *parentHashes.begin();
        setAncestors.insert(stageit);
        parentHashes.erase(stageit);
        totalSizeWithAncestors += stageit->GetTxSize();
        if (stageit->GetSizeWithDescendants() + entry.GetTxSize() > limitDescendantSize) {
            errString = strprintf("exceeds descendant size limit for tx %s [limit: %u]", stageit->GetTx().GetHash().ToString(), limitDescendantSize);
            return false;
        } else if (stageit->GetCountWithDescendants() + 1 > limitDescendantCount) {
            errString = strprintf("too many descendants for tx %s [limit: %u]", stageit->GetTx().GetHash().ToString(), limitDescendantCount);
            return false;
        } else if (totalSizeWithAncestors > limitAncestorSize) {
            errString = strprintf("exceeds ancestor size limit [limit: %u]", limitAncestorSize);
            return false;
        }
        const setEntries & setMemPoolParents = GetMemPoolParents(stageit);
        BOOST_FOREACH(const txiter &phash, setMemPoolParents) {
            
            if (setAncestors.count(phash) == 0) {
                parentHashes.insert(phash);
            }
            if (parentHashes.size() + setAncestors.size() + 1 > limitAncestorCount) {
                errString = strprintf("too many unconfirmed ancestors [limit: %u]", limitAncestorCount);
                return false;
            }
        }
    }
    return true;
}
void CTxMemPool::UpdateAncestorsOf(bool add, txiter it, setEntries &setAncestors)