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@@ -1040,23 +1040,21 @@ Front merges are handled by the binary trees in AS and deadline schedulers.
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iii. Plugging the queue to batch requests in anticipation of opportunities for
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merge/sort optimizations
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-This is just the same as in 2.4 so far, though per-device unplugging
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-support is anticipated for 2.5. Also with a priority-based i/o scheduler,
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-such decisions could be based on request priorities.
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-
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Plugging is an approach that the current i/o scheduling algorithm resorts to so
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that it collects up enough requests in the queue to be able to take
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advantage of the sorting/merging logic in the elevator. If the
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queue is empty when a request comes in, then it plugs the request queue
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-(sort of like plugging the bottom of a vessel to get fluid to build up)
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+(sort of like plugging the bath tub of a vessel to get fluid to build up)
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till it fills up with a few more requests, before starting to service
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the requests. This provides an opportunity to merge/sort the requests before
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passing them down to the device. There are various conditions when the queue is
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unplugged (to open up the flow again), either through a scheduled task or
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could be on demand. For example wait_on_buffer sets the unplugging going
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-(by running tq_disk) so the read gets satisfied soon. So in the read case,
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-the queue gets explicitly unplugged as part of waiting for completion,
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-in fact all queues get unplugged as a side-effect.
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+through sync_buffer() running blk_run_address_space(mapping). Or the caller
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+can do it explicity through blk_unplug(bdev). So in the read case,
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+the queue gets explicitly unplugged as part of waiting for completion on that
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+buffer. For page driven IO, the address space ->sync_page() takes care of
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+doing the blk_run_address_space().
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Aside:
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This is kind of controversial territory, as it's not clear if plugging is
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@@ -1067,11 +1065,6 @@ Aside:
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multi-page bios being queued in one shot, we may not need to wait to merge
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a big request from the broken up pieces coming by.
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- Per-queue granularity unplugging (still a Todo) may help reduce some of the
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- concerns with just a single tq_disk flush approach. Something like
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- blk_kick_queue() to unplug a specific queue (right away ?)
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- or optionally, all queues, is in the plan.
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-
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4.4 I/O contexts
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I/O contexts provide a dynamically allocated per process data area. They may
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be used in I/O schedulers, and in the block layer (could be used for IO statis,
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Jens Axboe