From: Eric Wong <e@80x24.org>
To: Jeff King <peff@peff.net>
Cc: Junio C Hamano <gitster@pobox.com>,
git@vger.kernel.org, "Shawn O. Pearce" <spearce@spearce.org>
Subject: Re: [PATCH] http-backend: buffer headers before sending
Date: Wed, 10 Aug 2016 20:36:11 +0000 [thread overview]
Message-ID: <20160810203611.GA24099@starla> (raw)
In-Reply-To: <20160810123201.ylfsnzmubpmtyoaa@sigill.intra.peff.net>
Jeff King <peff@peff.net> wrote:
> On Tue, Aug 09, 2016 at 11:47:31PM +0000, Eric Wong wrote:
>
> > Avoid waking up the readers for unnecessary context switches for
> > each line of header data being written, as all the headers are
> > written in short succession.
> >
> > It is unlikely any HTTP/1.x server would want to read a CGI
> > response one-line-at-a-time and trickle each to the client.
> > Instead, I'd expect HTTP servers want to minimize syscall and
> > TCP/IP framing overhead by trying to send all of its response
> > headers in a single syscall or even combining the headers and
> > first chunk of the body with MSG_MORE or writev.
> >
> > Verified by strace-ing response parsing on the CGI side.
>
> I don't think this is wrong to do, but it does feel like it makes the
> code slightly more brittle (you have to pass around the strbuf and
> remember to initialize it and end_headers() when you're done), for not
> much benefit.
Yes, I was nervous about some of these changes and had to look
it over several times. I think the problem was the code
initially assumed global state while this change localizes it;
so this ought to make future changes easier.
Thanks to you and Junio for the extra eyes.
> Using some kind of buffered I/O would be nicer, as then you would get
> nice-sized chunks without having to impact the code. I wonder if just
> using stdio here would be that bad. The place it usually sucks is in
> complex error handling, but we don't care about that at all here (I
> think we are basically happy to write until we get SIGPIPE).
stdio to a non-blocking pipe (if so setup by a caller) could be
problematic portability-wise. And reliance on SIGPIPE would
hurt reusability if this were eventually factored into a
standalone daemon using libmicrohttpd or similar.
> I dunno. I suspect the performance improvement from your patch is
> marginal, but it's not like the resulting code is all _that_ complex. So
> I guess I am OK either way, just not enthused.
Yes, marginal, but I still get annoyed to see extra lines from
strace (maybe I trace syscalls too much :x). But I think it's
also a baby step which opens up potential for the future. I
have nothing planned at the moment, but who knows in a year or
five...
> > ---
> > I admit I only noticed this because I was being lazy when
> > implementing the reader-side on an HTTP server by making
> > a single read(2) call :x
>
> The trouble is that your HTTP server is still broken. Now it's just
> broken in an unpredictable and racy way, because the OS may still split
> the write at PIPE_BUF boundaries. (Though given that this is not in the
> commit message, I suspect you know this patch is not an excuse not to
> fix your HTTP server).
Of course :)
next prev parent reply other threads:[~2016-08-10 20:36 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-08-09 23:47 [PATCH] http-backend: buffer headers before sending Eric Wong
2016-08-10 12:32 ` Jeff King
2016-08-10 20:36 ` Eric Wong [this message]
2016-08-10 16:27 ` Junio C Hamano
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