postgres-github.git / summary / log / commit / refs

commit    6e36930f9aaffd5e097a01935e6f68ed851535ae
Author:   Andres Freund <andres@anarazel.de>
Date:     Wed Apr 01 23:50:03 2026 +0000

    read_stream: Prevent distance from decaying too quickly
    
    Until now we reduced the look-ahead distance by 1 on every hit, and doubled it
    on every miss. That is problematic because there are very common IO patterns
    where this prevents us from ever reaching a sufficiently high distance (e.g. a
    miss followed by a hit will never have the distance grow beyond 2). In many
    such cases, if we had ever reached a sufficient look-ahead distance, things
    would have been fine, because we grow the distance faster than we decrease it.
    
    One might think that the most obvious answer to this problem would be to never
    reduce the distance. However, that would not work well, as (particularly with
    upcoming users of read streams), it is reasonably common to at first have a
    lot of misses and then to transition to a fully cached workload, e.g. because
    the same blocks are needed repeatedly within one stream. Doing unnecessarily
    deep readahead can be costly, due to having to pin a lot more buffers, which
    increases CPU overhead.
    
    Because the cost of a synchronously handled miss can be very high (multiple
    milliseconds for every IO with commonly used storage) compared to the CPU
    overhead of keeping the distance too high, we want to err on the side of not
    reducing the distance too early.
    
    The insight that a decrease of the distance by 1 at ever hit may be ok at
    large distances, but not at low distances, shows a way out: If we only allow
    decreasing the distance once there were no misses for our maximum look-ahead
    distance, we will keep the distance high as long as readahead has a chance to
    do IO asynchronously, but not commonly when not.
    
    Several folks have written variants of this patch, including at least Thomas
    Munro, Melanie Plageman and I.
    
    Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
    Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
    Discussion: https://postgr.es/m/f3xxfrkafjxpyqxywcxricxgyizjirfceychyxsgn7bwjp5eda@kwbduhy7tfmu
    Discussion: https://postgr.es/m/CA+hUKGL2PhFyDoqrHefqasOnaXhSg48t1phs3VM8BAdrZqKZkw@mail.gmail.com
    Discussion: https://postgr.es/m/CAH2-Wz%3DkMg3PNay96cHMT0LFwtxP-cQSRZTZzh1Cixxf8G%3Dzrw%40mail.gmail.com

[parent: cceb1bf45e3a]