• Kirill Smelkov's avatar
    go/zodb: LiveCache: Allow objects to be pinned / omitted from the cache · 4357db3f
    Kirill Smelkov authored
    We already have LiveCacheControl and policy to keep object state in live
    cache. However that state is currently kept only until the object is
    present in the cache, and the object is there currently only until there
    are live pointers to the object from anywhere. If all such pointers go
    away, LiveCache currently discards the object.
    
    Add a way (PCachePinObject) to tell live cache that it must retain an
    object even if there are no other pointers to it. This is needed for
    Wendelin.core which semantically relies on some objects to be present in
    live cache, even if in ghost state, for propagating ZODB invalidations
    into OS pagecache:
    
    https://lab.nexedi.com/kirr/wendelin.core/blob/000bf16359/wcfs/wcfs.go#L245
    
    For symmetry add a way (PCacheDropObject) to tell live cache that it
    must not retain an object. This makes sense for objects that are read in
    large only once to avoid needlessly evicting all other objects while
    making the room for what won't be soon used again.
    
    For completeness add also a way (PCacheDropState) to tell live cache
    that the state of this object should not be cached. This can make sense
    even for objects that are pinned: for example Wendelin.core pins ZBlk
    objects to rely on volatile metadata it attaches to them, but have to
    drop ZBlk state that was loaded from database: that state is propagated
    to be cached in OS pagecache, and keeping it also in ZODB live cache
    would just waste RAM.
    
    Finally add test to cover LiveCache/LiveCacheControl functionality in
    detail.
    4357db3f
connection.go 11.7 KB