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AndresFreundTec's profile
Andres Freund (Tech)
Andres Freund (Tech)
Andres Freund (Tech)
@AndresFreundTec

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Andres Freund (Tech)

@AndresFreundTec

Postgres developer, working at Microsoft. Also: politics nerd, expat German in the US. Account for tech related things. For politics: @AndresFreundPol

San Francisco, CA
Joined August 2017

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    1. Andres Freund (Tech)‏ @AndresFreundTec Feb 23
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      Replying to @axboe

      IORING_OP_BUFFER_FREE suggests that buffers are registered somehow (?). How about instead having an explicit opcode that provides a buffer for a one-off use? A IORING_OP_PROVIDE_BUFFER registers a buffer as available, with buf_index (under different name) indicating a 'group'.

      2 replies 0 retweets 0 likes
    2. Jens Axboe‏ @axboe Feb 23
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      Replying to @AndresFreundTec

      Question is where that buffer comes from, my current code is having the app provide them (upfront). Kernel selects one (for IOSQE_BUFFER_SELECT), and IORING_OP_BUFFER_FREE marks that buffer as free again.

      1 reply 0 retweets 0 likes
    3. Andres Freund (Tech)‏ @AndresFreundTec Feb 23
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      Replying to @axboe

      Was thinking that buffers are provided one-off, not permanently. So you'd have to IORING_OP_PROVIDE_BUFFER once for each buffer ahead of time, and then *again* after used by kernel and consumed by userland. Don't see a lot of benefit of permanent in kernel registry.

      1 reply 0 retweets 0 likes
    4. Jens Axboe‏ @axboe Feb 23
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      Replying to @AndresFreundTec

      But how would that work for tons of connections, where you generally see poll + recv done now? We can't arm tons of recv with PROVIDE_BUFFER, I don't see how that improves the situation, we'd still need X buffers for X connections. What am I missing?

      1 reply 0 retweets 0 likes
    5. Andres Freund (Tech)‏ @AndresFreundTec Feb 23
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      Replying to @axboe

      Oh. What I mean is that there's several potential *sets* of ready buffers (identified by a new union member alongside buffer_index). A RECVMSG etc specifies the set to use (buffer_index = registered_buffer = 17). If no buffer in set once ready, signal EBUSY.

      2 replies 0 retweets 0 likes
    6. Andres Freund (Tech)‏ @AndresFreundTec Feb 23
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      Replying to @AndresFreundTec @axboe

      Once userspace is done with a buffer used by RECVMSG, it can free the memory, or provide it again to the kernel for the set of buffers. Multiple sets because different recvmsgs might need differently sized buffers.

      1 reply 0 retweets 0 likes
    7. Andres Freund (Tech)‏ @AndresFreundTec Feb 23
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      Replying to @AndresFreundTec @axboe

      E.g. the sockets waiting for a new 'command' from client wouldn't want a large buffer, but sockets waiting for a bulk file upload would.

      1 reply 0 retweets 2 likes
    8. Jens Axboe‏ @axboe Feb 23
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      Replying to @AndresFreundTec

      Totally untested, but see below for the direction. What do you think? https://git.kernel.dk/cgit/linux-block/log/?h=io_uring-buf-select …

      1 reply 0 retweets 1 like
    9. Andres Freund (Tech)‏ @AndresFreundTec Feb 23
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      Replying to @axboe

      Looks good on a first read. Probably need to limit buffer ids to something < 32bit? Should the kernel check for non-uniqueness of ids?

      1 reply 0 retweets 0 likes
    10. Jens Axboe‏ @axboe Feb 23
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      Replying to @AndresFreundTec

      Pushed update, now it works. No error checking is done yet, for length of buffer, group id, etc. So that needs doing, this is just provided for direction. I thought about the buffer id, and I think that's just up to the application. But I'm open to improvements.

      1 reply 0 retweets 0 likes
      Andres Freund (Tech)‏ @AndresFreundTec Feb 23
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      Replying to @axboe

      Mostly commented on the buffer ids width bit because it's ABI. Wondering whether cqe.flags ought to be split into two parts. Perhaps that'd still be ok because there's no uses of it yet?

      4:42 PM - 23 Feb 2020
      1 reply 0 retweets 0 likes
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        2. Jens Axboe‏ @axboe Feb 23
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          Replying to @AndresFreundTec

          Not close enough to worry about ABI yet ;-) We could split the flags, it is indeed unused. But 2G buffer IDs is probably enough? And leaves us free to use the flags in the future as well.

          1 reply 0 retweets 0 likes
        3. Andres Freund (Tech)‏ @AndresFreundTec Feb 23
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          Replying to @axboe

          For future extensibility it seems like it'd be good to have space for additional cqe flags that can be specified concurrently with IORING_CQE_F_BUFFER, Which'd be harder if 31 bit buffer ids were allowed initially. Hence wondering about a more explicit split of flag & "tag".

          1 reply 0 retweets 1 like
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