diff options
| -rw-r--r-- | src/backend/storage/aio/read_stream.c | 68 |
1 files changed, 16 insertions, 52 deletions
diff --git a/src/backend/storage/aio/read_stream.c b/src/backend/storage/aio/read_stream.c index 9c0163eda3e..d65fa07b44c 100644 --- a/src/backend/storage/aio/read_stream.c +++ b/src/backend/storage/aio/read_stream.c @@ -17,30 +17,12 @@ * pending read. When that isn't possible, the existing pending read is sent * to StartReadBuffers() so that a new one can begin to form. * - * The algorithm for controlling the look-ahead distance tries to classify the - * stream into three ideal behaviors: - * - * A) No I/O is necessary, because the requested blocks are fully cached - * already. There is no benefit to looking ahead more than one block, so - * distance is 1. This is the default initial assumption. - * - * B) I/O is necessary, but read-ahead advice is undesirable because the - * access is sequential and we can rely on the kernel's read-ahead heuristics, - * or impossible because direct I/O is enabled, or the system doesn't support - * read-ahead advice. There is no benefit in looking ahead more than - * io_combine_limit, because in this case the only goal is larger read system - * calls. Looking further ahead would pin many buffers and perform - * speculative work for no benefit. - * - * C) I/O is necessary, it appears to be random, and this system supports - * read-ahead advice. We'll look further ahead in order to reach the - * configured level of I/O concurrency. - * - * The distance increases rapidly and decays slowly, so that it moves towards - * those levels as different I/O patterns are discovered. For example, a - * sequential scan of fully cached data doesn't bother looking ahead, but a - * sequential scan that hits a region of uncached blocks will start issuing - * increasingly wide read calls until it plateaus at io_combine_limit. + * The algorithm for controlling the look-ahead distance is based on recent + * cache hit and miss history. When no I/O is necessary, there is no benefit + * in looking ahead more than one block. This is the default initial + * assumption, but when blocks needing I/O are streamed, the distance is + * increased rapidly to try to benefit from I/O combining and concurrency. It + * is reduced gradually when cached blocks are streamed. * * The main data structure is a circular queue of buffers of size * max_pinned_buffers plus some extra space for technical reasons, ready to be @@ -333,7 +315,7 @@ read_stream_start_pending_read(ReadStream *stream) /* Remember whether we need to wait before returning this buffer. */ if (!need_wait) { - /* Look-ahead distance decays, no I/O necessary (behavior A). */ + /* Look-ahead distance decays, no I/O necessary. */ if (stream->distance > 1) stream->distance--; } @@ -634,7 +616,7 @@ read_stream_begin_impl(int flags, /* * Skip the initial ramp-up phase if the caller says we're going to be * reading the whole relation. This way we start out assuming we'll be - * doing full io_combine_limit sized reads (behavior B). + * doing full io_combine_limit sized reads. */ if (flags & READ_STREAM_FULL) stream->distance = Min(max_pinned_buffers, stream->io_combine_limit); @@ -725,10 +707,10 @@ read_stream_next_buffer(ReadStream *stream, void **per_buffer_data) #ifndef READ_STREAM_DISABLE_FAST_PATH /* - * A fast path for all-cached scans (behavior A). This is the same as the - * usual algorithm, but it is specialized for no I/O and no per-buffer - * data, so we can skip the queue management code, stay in the same buffer - * slot and use singular StartReadBuffer(). + * A fast path for all-cached scans. This is the same as the usual + * algorithm, but it is specialized for no I/O and no per-buffer data, so + * we can skip the queue management code, stay in the same buffer slot and + * use singular StartReadBuffer(). */ if (likely(stream->fast_path)) { @@ -848,28 +830,10 @@ read_stream_next_buffer(ReadStream *stream, void **per_buffer_data) if (++stream->oldest_io_index == stream->max_ios) stream->oldest_io_index = 0; - if (stream->ios[io_index].op.flags & READ_BUFFERS_ISSUE_ADVICE) - { - /* Distance ramps up fast (behavior C). */ - distance = stream->distance * 2; - distance = Min(distance, stream->max_pinned_buffers); - stream->distance = distance; - } - else - { - /* No advice; move towards io_combine_limit (behavior B). */ - if (stream->distance > stream->io_combine_limit) - { - stream->distance--; - } - else - { - distance = stream->distance * 2; - distance = Min(distance, stream->io_combine_limit); - distance = Min(distance, stream->max_pinned_buffers); - stream->distance = distance; - } - } + /* Look-ahead distance ramps up rapidly after we do I/O. */ + distance = stream->distance * 2; + distance = Min(distance, stream->max_pinned_buffers); + stream->distance = distance; /* * If we've reached the first block of a sequential region we're |