-- | The event manager state.
data EventManager = EventManager
    { emBackend      :: !Backend
    , emFds          :: {-# UNPACK #-} !(Array Int (MVar (IntTable [FdData])))
    , emState        :: {-# UNPACK #-} !(IORef State)
    , emUniqueSource :: {-# UNPACK #-} !UniqueSource
    , emControl      :: {-# UNPACK #-} !Control
    , emLock         :: {-# UNPACK #-} !(MVar ())
    }

the field in question is an Array Int (MVar (IntTable [FdData])).

EventManager could instead be:

type MVarIO = MVar# RealWorld -- type synonym for brevity

-- | The event manager state.
data EventManager = EventManager
    { emBackend      :: !Backend
    , emFds          :: {-# UNPACK #-} !(UnliftedArray (MVarIO (IntTable [FdData])))
    , emState        :: {-# UNPACK #-} !(IORef State)
    , emUniqueSource :: {-# UNPACK #-} !UniqueSource
    , emControl      :: {-# UNPACK #-} !Control
    , emLock         :: {-# UNPACK #-} !(MVar ())
    }

now the UnliftedArray contains non-thunks.