Messages in this thread | | | Subject | Re: [PATCH] pipe: wakeup writer only if pipe buffer is at least half empty | From | Konstantin Khlebnikov <> | Date | Mon, 28 Oct 2019 12:09:01 +0300 |
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On 27/10/2019 19.12, Linus Torvalds wrote: > On Sun, Oct 27, 2019 at 11:46 AM Konstantin Khlebnikov > <khlebnikov@yandex-team.ru> wrote: >> >> There is no reason to wakeup writer if pipe has only one empty page. >> This means reader consumes data slower then writer produces it. >> >> This patch waits until buffer is at least half empty before waking writer. > > This is a bit dangerous, at least with David's other changes. > > In particular, there's now a "max_usage" in his series means that the > writer might be blocked even if there's lots of free slots, because > the writer is only allowed to use part of those slots. > > So I'd rather not see this logic particularly now that David is > working on modifying the overall pipe logic. > > I do agree with the overall idea, but I'm not entirely happy about the > "half full" logic, because it gets subtle with David's changes. > > Also, I'm a bit worried about cases where the readers and writers > block on each other, and depend on "there's enough space in the pipe > that we won't deadlock". Maybe the writer is blocked (because it > filled the pipe), the reader reads just part of the pipe, and then the > reader blocks on the writer doing something else, knowing that it just > free'd up resources for the writer. But the writer is still blocked, > and not woken up, because the pipe is still more than half full. See > what I'm saying? > > I'm not sure anything like that exists, but it's an example of a "hmm" > condition. >
Ok. This breakage scenario is doubtful but such weird software really might exist.
What about making this thing tunable via fcntl like size of pipe buffer?
fcntl(fd, F_SETPIPE_WRITE_SZ, size)
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