Building 2dsphere index uses excessive memory

Create a collection with 24 million documents {loc: [0, 0]}. Then create an index with db.c.createIndex({loc:"2dsphere"}).

• index create runs from A to B
• 1825 MB allocated outside the cache ("allocated minus wt cache")
• heap profile shows the following stack accounts for most of the allocations:

  heapProfile stack39: { 0: "tc_malloc", 1: "mongo::mongoMalloc", 2: "mongo::S2CellIdToIndexKey", 3: "0xc79010", 4: "mongo::ExpressionKeysPrivate::getS2Keys", 5: "mongo::IndexAccessMethod::BulkBuilder::insert", 6: "mongo::MultiIndexBlock::insert", 7: "mongo::MultiIndexBlock::insertAllDocumentsInCollection", 8: "mongo::CmdCreateIndex::run", 9: "mongo::Command::run", 10: "mongo::Command::execCommand", 11: "mongo::runCommands", 12: "mongo::assembleResponse", 13: "mongo::MyMessageHandler::process", 14: "mongo::PortMessageServer::handleIncomingMsg", 15: "0x7fd30dc996aa", 16: "clone" }

  These are keys being accumulated for sorting. The accumulated size of the keys is growing to 1348 MiB before external sort is used whereas it should be limited to 100 MiB.