From: Ken Brown <kbrown@cornell.edu>
To: cygwin-developers@cygwin.com
Subject: Re: cygrunsrv + sshd + rsync = 20 times too slow -- throttled?
Date: Thu, 2 Sep 2021 09:01:02 -0400 [thread overview]
Message-ID: <152bfc0c-2f72-c684-6fc5-aa7c36c136b8@cornell.edu> (raw)
In-Reply-To: <YTCIqrhZ95MZ1teQ@calimero.vinschen.de>
On 9/2/2021 4:17 AM, Corinna Vinschen wrote:
> On Sep 1 19:02, Ken Brown wrote:
>> On 9/1/2021 9:52 AM, Corinna Vinschen wrote:
>>> Great idea that. What we need would be some semaphore upside down.
>>> One that can be used to count items and which is signalled if it's
>>> down to zero.
>>
>> Here's an idea (untested), based on https://stackoverflow.com/questions/6559854/is-there-something-opposite-to-semaphore:
>>
>> We create an ordinary Windows semaphore and use it to count the readers: It
>> starts at 0, we increment it by calling ReleaseSemaphore when a reader is
>> opened (by fhandler_pipe::create, fork/exec, dup), and we decrement it by
>> calling WFSO when a reader closes. When we decrement it, we test whether
>> it's been reduced to 0. We do this by calling ReleaseSemaphore and using
>> its lpPreviousCount argument.
>>
>> We also create an event that we can use to make WFMO return during a
>> blocking write. We signal this event if a reader closes and we've
>> discovered that there are no more readers. In this case we cancel the
>> pending write [*] and return an error.
>>
>> I'm sure I've overlooked something, but does this seem feasible?
>
> It could work, but the problem with all these approaches is that they
> are tricky and bound to fail as soon as a process is killed or crashes.
>
>> [*] I don't know offhand if Windows provides a way to cancel a pending
>> write. If not, we could use query_hdl to drain the pipe.
>
> There's a CancelIoEx function to cancel all async IO on a handle.
>
> In a lucid moment tonight, I had another idea.
>
> First of all, scratch my patch. Also, revert select to check only
> for WriteQuotaAvailable.
>
> Next, for sanity, let's assume that non-blocking reads don't change
> WriteQuotaAvailable. So the only important case is the blocking read,
> which reduces WriteQuotaAvailable by the number of requested bytes.
>
> Next, fact is, we're only interested in WriteQuotaAvailable > 0.
> And we have a buffersize of 64K.
>
> We can also safely assume that we only have a very small number of
> readers, typically only one.
>
> So here's the crazily simple idea:
>
> What if the readers never request more than, say, 50 or even 25% of the
> available buffer space? Our buffer is 64K and there's no guarantee that
> any read > PIPE_BUF (== 4K) is atomic anyway. This can work without
> having to check the other side of the pipe. Something like this,
> ignoring border cases:
>
> pipe::create()
> {
> [...]
> mutex = CreateMutex();
> }
>
> pipe::raw_read(char *buf, size_t num_requested)
> {
> if (blocking)
> {
> WFSO(mutex);
> NtQueryInformationFile(FilePipeLocalInformation);
> if (!fpli.ReadDataAvailable
> && num_requested > fpli.InboundQuota / 4)
> num_requested = fpli.InboundQuota / 4;
> NtReadFile(pipe, buf, num_requested);
> ReleaseMutex(mutex);
> }
> }
>
> It's not entirely foolproof, but it should fix 99% of the cases.
I like it!
Do you think there's anything we can or should do to avoid a deadlock in the
rare cases where this fails? The only thing I can think of immediately is to
always impose a timeout if select is called with infinite timeout on the write
side of a pipe, after which we report that the pipe is write ready. After all,
we've lived since 2008 with a bug that caused select to *always* report write ready.
Alternatively, we could just wait and see if there's an actual use case in which
someone encounters a deadlock.
Ken
next prev parent reply other threads:[~2021-09-02 13:01 UTC|newest]
Thread overview: 250+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <41A583E1-C8E7-42AB-9F24-EEC33A41EC60@house.org>
[not found] ` <20210825201845.07b6400b79dc5558a7761efe@nifty.ne.jp>
[not found] ` <f8106fe7-a2b8-d106-3061-4d888124f4b0@cornell.edu>
[not found] ` <20210826062934.54f2f2216021c095bb7ba13b@nifty.ne.jp>
[not found] ` <d0a8c57d-1ed1-6b4f-c6e7-cbe0e2ec8a1c@cornell.edu>
[not found] ` <3b560051-ab27-f392-ca4b-d1fd9b5733b0@cornell.edu>
[not found] ` <20210827202440.47706fc2fc07c5e9a1bc0047@nifty.ne.jp>
[not found] ` <4f2cb5f3-ce9c-c617-f65f-841a5eca096e@cornell.edu>
[not found] ` <20210828022111.91ef5b4ff24f6da9fadb489e@nifty.ne.jp>
[not found] ` <YSn3L0W1M527utK0@calimero.vinschen.de>
[not found] ` <20210828184102.f2206a8a9e5fe5cf24bf5e45@nifty.ne.jp>
[not found] ` <YSok0PoCQn2TPPrn@calimero.vinschen.de>
[not found] ` <20210829004346.c2f80469abc3a07fd4b2918d@nifty.ne.jp>
[not found] ` <e8caa02f-be85-33bc-3f09-347c1cdb0923@cornell.edu>
[not found] ` <20210829174124.0c1ae6c16a3e8da1f490abc7@nifty.ne.jp>
[not found] ` <6e9bb35e-6f4f-cf78-e515-549da487b5ef@cornell.edu>
2021-08-30 7:57 ` Corinna Vinschen
[not found] ` <20210829180729.48b4e877f773cb3980c5766d@nifty.ne.jp>
[not found] ` <789f056a-f164-d71d-1dc9-230f5a41846d@cornell.edu>
2021-08-30 8:27 ` Corinna Vinschen
2021-08-30 13:00 ` Corinna Vinschen
2021-08-30 13:20 ` Corinna Vinschen
2021-08-30 13:41 ` Ken Brown
2021-08-30 14:12 ` Corinna Vinschen
2021-08-30 14:52 ` Ken Brown
2021-08-30 15:15 ` Corinna Vinschen
[not found] ` <20210830043756.8aa0ada77db0bfbbe3889f62@nifty.ne.jp>
[not found] ` <47e5dd74-b940-f305-fd5a-c6c9d8f41305@cornell.edu>
2021-08-30 8:48 ` Corinna Vinschen
[not found] ` <c62d18df-ab7a-7233-62ee-29a8eced5353@cornell.edu>
[not found] ` <20210830091314.f9a2cb71794d0f68cdb5eba7@nifty.ne.jp>
[not found] ` <20210830092259.52f7d54fc3fa340738373af4@nifty.ne.jp>
[not found] ` <d217ef03-7858-5e22-0aa6-f0507eedd9da@cornell.edu>
[not found] ` <20210830170204.fa91eaf110f310f13b67abc3@nifty.ne.jp>
2021-08-30 10:20 ` Corinna Vinschen
2021-08-30 10:38 ` Corinna Vinschen
2021-08-30 12:04 ` Takashi Yano
2021-08-30 12:55 ` Corinna Vinschen
2021-08-30 13:31 ` Corinna Vinschen
2021-08-31 8:50 ` Takashi Yano
2021-08-30 13:51 ` Ken Brown
2021-08-30 15:00 ` Ken Brown
2021-08-30 15:19 ` Corinna Vinschen
2021-08-30 15:43 ` Ken Brown
2021-08-31 9:43 ` Corinna Vinschen
2021-08-31 8:52 ` Takashi Yano
2021-08-31 9:04 ` Corinna Vinschen
2021-08-31 11:05 ` Takashi Yano
2021-08-31 15:20 ` Corinna Vinschen
2021-09-01 2:39 ` Takashi Yano
2021-09-01 8:03 ` Corinna Vinschen
2021-09-01 8:13 ` Corinna Vinschen
2021-08-30 13:36 ` Ken Brown
2021-08-30 14:05 ` Corinna Vinschen
2021-08-30 15:53 ` Corinna Vinschen
2021-08-30 17:00 ` Corinna Vinschen
2021-08-30 17:11 ` Corinna Vinschen
2021-08-30 18:59 ` Ken Brown
2021-08-30 19:12 ` Ken Brown
2021-08-30 20:21 ` Corinna Vinschen
2021-08-30 20:14 ` Corinna Vinschen
2021-08-30 20:47 ` Ken Brown
2021-08-31 8:55 ` Takashi Yano
2021-08-31 9:08 ` Corinna Vinschen
2021-08-31 9:25 ` Takashi Yano
2021-08-31 10:05 ` Corinna Vinschen
2021-08-31 10:18 ` Corinna Vinschen
2021-08-31 11:45 ` Takashi Yano
2021-08-31 12:31 ` Takashi Yano
2021-08-31 15:08 ` Corinna Vinschen
2021-08-31 12:33 ` Ken Brown
2021-08-31 15:18 ` Corinna Vinschen
2021-08-31 15:27 ` Corinna Vinschen
2021-08-31 15:50 ` Corinna Vinschen
2021-08-31 16:19 ` Ken Brown
2021-08-31 16:38 ` Ken Brown
2021-08-31 17:30 ` Corinna Vinschen
2021-08-31 18:54 ` Ken Brown
2021-08-31 19:51 ` Corinna Vinschen
2021-08-31 23:02 ` Takashi Yano
2021-09-01 0:16 ` Takashi Yano
2021-09-01 8:07 ` Corinna Vinschen
2021-09-01 8:23 ` Takashi Yano
2021-09-01 8:46 ` Corinna Vinschen
2021-09-01 12:56 ` Ken Brown
2021-09-01 13:52 ` Corinna Vinschen
2021-09-01 23:02 ` Ken Brown
2021-09-02 8:17 ` Corinna Vinschen
2021-09-02 13:01 ` Ken Brown [this message]
2021-09-02 19:00 ` Corinna Vinschen
2021-09-02 19:34 ` Ken Brown
2021-09-02 19:35 ` Corinna Vinschen
2021-09-02 20:19 ` Ken Brown
2021-09-03 9:12 ` Corinna Vinschen
2021-09-03 19:00 ` Ken Brown
2021-09-03 19:53 ` Ken Brown
2021-09-03 19:54 ` Corinna Vinschen
2021-09-03 20:05 ` Ken Brown
2021-09-03 10:00 ` Takashi Yano
2021-09-03 10:13 ` Takashi Yano
2021-09-03 11:31 ` Corinna Vinschen
2021-09-03 11:41 ` Corinna Vinschen
2021-09-03 12:13 ` Ken Brown
2021-09-03 15:00 ` Corinna Vinschen
2021-09-03 15:14 ` Ken Brown
2021-09-03 15:17 ` Corinna Vinschen
2021-09-03 12:22 ` Takashi Yano
2021-09-03 13:27 ` Ken Brown
2021-09-03 15:37 ` Corinna Vinschen
2021-09-04 12:02 ` Takashi Yano
2021-09-04 12:37 ` Takashi Yano
2021-09-04 14:04 ` Ken Brown
2021-09-04 23:15 ` Takashi Yano
2021-09-05 13:40 ` Takashi Yano
2021-09-05 13:50 ` Takashi Yano
2021-09-05 18:47 ` Ken Brown
2021-09-05 19:42 ` Takashi Yano
2021-09-05 20:09 ` Takashi Yano
2021-09-05 20:27 ` Ken Brown
2021-09-06 8:13 ` Corinna Vinschen
2021-09-06 11:16 ` Takashi Yano
2021-09-06 12:49 ` Corinna Vinschen
2021-09-06 13:16 ` Takashi Yano
2021-09-06 16:08 ` Corinna Vinschen
2021-09-06 23:39 ` Takashi Yano
2021-09-07 9:14 ` Corinna Vinschen
2021-09-07 11:03 ` Takashi Yano
2021-09-07 16:14 ` Ken Brown
2021-09-07 18:26 ` Corinna Vinschen
2021-09-03 10:38 ` Takashi Yano
2021-09-08 11:32 ` Takashi Yano
2021-09-08 11:55 ` Corinna Vinschen
2021-09-08 12:33 ` Takashi Yano
2021-09-08 17:43 ` Ken Brown
2021-09-08 18:28 ` Corinna Vinschen
2021-09-02 8:15 ` Takashi Yano
2021-09-02 18:54 ` Corinna Vinschen
2021-09-07 3:26 ` Takashi Yano
2021-09-07 10:50 ` Takashi Yano
2021-09-08 0:07 ` Takashi Yano
2021-09-08 4:11 ` Takashi Yano
2021-09-08 9:01 ` Takashi Yano
2021-09-08 9:01 ` Corinna Vinschen
2021-09-08 9:26 ` Corinna Vinschen
2021-09-08 9:45 ` Takashi Yano
2021-09-08 10:04 ` Corinna Vinschen
2021-09-08 10:45 ` Takashi Yano
2021-09-08 10:51 ` Corinna Vinschen
2021-09-09 3:21 ` Takashi Yano
2021-09-09 9:37 ` Corinna Vinschen
2021-09-09 10:55 ` Takashi Yano
2021-09-09 11:41 ` Corinna Vinschen
2021-09-08 9:37 ` Takashi Yano
2021-09-09 3:41 ` Takashi Yano
2021-09-09 8:05 ` Takashi Yano
2021-09-09 12:19 ` Takashi Yano
2021-09-09 12:42 ` Takashi Yano
2021-09-09 21:53 ` Takashi Yano
2021-09-10 3:41 ` Takashi Yano
2021-09-10 10:57 ` Ken Brown
2021-09-10 15:17 ` Ken Brown
2021-09-10 15:26 ` Corinna Vinschen
2021-09-10 22:57 ` Takashi Yano
2021-09-11 2:17 ` Ken Brown
2021-09-11 2:35 ` Takashi Yano
2021-09-11 13:12 ` Ken Brown
2021-09-12 6:23 ` Takashi Yano
2021-09-12 14:39 ` Ken Brown
2021-09-13 9:11 ` Corinna Vinschen
2021-09-13 12:30 ` Ken Brown
2021-09-12 8:48 ` Takashi Yano
2021-09-12 11:04 ` Takashi Yano
2021-09-12 15:10 ` Ken Brown
2021-09-12 21:46 ` Ken Brown
2021-09-12 23:54 ` Takashi Yano
2021-09-13 2:19 ` Ken Brown
2021-09-13 8:40 ` Takashi Yano
2021-09-13 12:51 ` Ken Brown
2021-09-13 17:05 ` Ken Brown
2021-09-13 9:42 ` Corinna Vinschen
2021-09-13 13:03 ` Ken Brown
2021-09-13 18:39 ` Takashi Yano
2021-09-12 23:41 ` Takashi Yano
2021-09-13 17:42 ` Ken Brown
2021-09-13 18:54 ` Takashi Yano
2021-09-13 18:32 ` Corinna Vinschen
2021-09-13 19:37 ` Takashi Yano
2021-09-13 20:15 ` Corinna Vinschen
2021-09-14 8:07 ` Takashi Yano
2021-09-14 8:47 ` Corinna Vinschen
2021-09-14 12:38 ` Ken Brown
2021-09-14 14:15 ` Corinna Vinschen
2021-09-14 8:08 ` Takashi Yano
2021-09-14 9:03 ` Corinna Vinschen
2021-09-14 9:56 ` Takashi Yano
2021-09-14 10:19 ` Takashi Yano
2021-09-14 11:03 ` Corinna Vinschen
2021-09-14 12:05 ` Takashi Yano
2021-09-14 14:17 ` Corinna Vinschen
2021-09-14 22:14 ` Ken Brown
2021-09-15 0:21 ` Takashi Yano
2021-09-15 0:44 ` Takashi Yano
2021-09-15 0:59 ` Takashi Yano
2021-09-15 9:57 ` Corinna Vinschen
2021-09-15 10:48 ` Takashi Yano
2021-09-15 10:58 ` Takashi Yano
2021-09-15 11:34 ` Corinna Vinschen
2021-09-15 11:40 ` Corinna Vinschen
2021-09-15 11:13 ` Corinna Vinschen
2021-09-15 11:41 ` Ken Brown
2021-09-15 11:49 ` Corinna Vinschen
2021-09-15 11:54 ` Takashi Yano
2021-09-15 12:20 ` Corinna Vinschen
2021-09-15 13:04 ` Takashi Yano
2021-09-15 13:42 ` Corinna Vinschen
2021-09-15 16:22 ` Ken Brown
2021-09-15 17:09 ` Ken Brown
2021-09-16 0:22 ` Takashi Yano
2021-09-16 2:28 ` Ken Brown
2021-09-16 9:09 ` Takashi Yano
2021-09-16 13:02 ` Takashi Yano
2021-09-16 13:25 ` Corinna Vinschen
2021-09-16 14:27 ` Takashi Yano
2021-09-16 15:01 ` Corinna Vinschen
2021-09-16 15:46 ` Ken Brown
2021-09-16 16:02 ` Ken Brown
2021-09-16 19:42 ` Takashi Yano
2021-09-16 20:28 ` Ken Brown
2021-09-16 19:48 ` Ken Brown
2021-09-16 20:01 ` Takashi Yano
2021-09-17 2:25 ` Ken Brown
2021-09-17 8:31 ` Takashi Yano
2021-09-17 11:16 ` Ken Brown
2021-09-17 16:23 ` Takashi Yano
2021-09-17 17:08 ` Ken Brown
2021-09-17 17:39 ` Jon Turney
2021-09-17 17:43 ` Takashi Yano
2021-09-17 19:53 ` Ken Brown
2021-09-18 1:30 ` Takashi Yano
2021-09-18 2:07 ` Ken Brown
2021-09-18 2:10 ` Ken Brown
2021-09-18 8:03 ` Takashi Yano
2021-09-18 11:12 ` Ken Brown
2021-09-18 11:35 ` Takashi Yano
2021-09-18 14:11 ` Jon Turney
2021-09-18 13:44 ` Ken Brown
2021-09-19 1:31 ` Takashi Yano
2021-09-19 14:35 ` Ken Brown
2021-09-20 9:29 ` Takashi Yano
2021-09-16 0:13 ` Takashi Yano
2021-09-16 2:26 ` Ken Brown
2021-09-13 9:07 ` Corinna Vinschen
2021-09-20 12:52 ` Takashi Yano
2021-09-20 19:14 ` Ken Brown
2021-09-20 21:09 ` Ken Brown
2021-09-20 21:21 ` Ken Brown
2021-09-20 21:27 ` Takashi Yano
2021-09-20 21:39 ` Ken Brown
2021-09-20 22:16 ` Takashi Yano
2021-09-20 22:46 ` Ken Brown
2021-09-20 22:50 ` Ken Brown
2021-09-20 23:22 ` Takashi Yano
2021-09-21 8:30 ` Takashi Yano
2021-09-21 9:26 ` Mark Geisert
2021-09-21 10:10 ` Takashi Yano
2021-09-21 21:10 ` Mark Geisert
2021-09-21 13:31 ` Ken Brown
2021-09-21 15:36 ` Takashi Yano
2021-09-21 18:51 ` Ken Brown
2021-09-23 8:26 ` Takashi Yano
2021-09-23 13:03 ` Ken Brown
2021-09-23 15:03 ` Takashi Yano
2021-09-23 16:29 ` Ken Brown
2021-10-18 10:51 ` Corinna Vinschen
2021-10-18 12:02 ` Takashi Yano
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