Return-Path: X-Original-To: ietfarch-httpbisa-archive-bis2Juki@ietfa.amsl.com Delivered-To: ietfarch-httpbisa-archive-bis2Juki@ietfa.amsl.com Received: from localhost (localhost [127.0.0.1]) by ietfa.amsl.com (Postfix) with ESMTP id 9BF8521F8BC0 for ; Tue, 28 May 2013 18:13:59 -0700 (PDT) X-Virus-Scanned: amavisd-new at amsl.com X-Spam-Flag: NO X-Spam-Score: -10.388 X-Spam-Level: X-Spam-Status: No, score=-10.388 tagged_above=-999 required=5 tests=[AWL=-0.090, BAYES_00=-2.599, HTML_MESSAGE=0.001, MIME_8BIT_HEADER=0.3, RCVD_IN_DNSWL_HI=-8] Received: from mail.ietf.org ([12.22.58.30]) by localhost (ietfa.amsl.com [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id RdY93QtLSI6F for ; Tue, 28 May 2013 18:13:52 -0700 (PDT) Received: from frink.w3.org (frink.w3.org [128.30.52.56]) by ietfa.amsl.com (Postfix) with ESMTP id 6834221F842B for ; Tue, 28 May 2013 18:13:52 -0700 (PDT) Received: from lists by frink.w3.org with local (Exim 4.72) (envelope-from ) id 1UhUxM-0000A1-Rx for ietf-http-wg-dist@listhub.w3.org; Wed, 29 May 2013 01:13:36 +0000 Resent-Date: Wed, 29 May 2013 01:13:36 +0000 Resent-Message-Id: Received: from lisa.w3.org ([128.30.52.41]) by frink.w3.org with esmtp (Exim 4.72) (envelope-from ) id 1UhUxA-00007h-E0 for ietf-http-wg@listhub.w3.org; Wed, 29 May 2013 01:13:24 +0000 Received: from mail-oa0-f49.google.com ([209.85.219.49]) by lisa.w3.org with esmtps (TLS1.0:RSA_ARCFOUR_SHA1:16) (Exim 4.72) (envelope-from ) id 1UhUx8-0003mS-U4 for ietf-http-wg@w3.org; Wed, 29 May 2013 01:13:24 +0000 Received: by mail-oa0-f49.google.com with SMTP id k14so10659610oag.8 for ; Tue, 28 May 2013 18:12:57 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :cc:content-type; bh=7QhfGTPx570npH9KGf6h3D4tVRlNxfRB1O0jvj17TXE=; b=xojxBwbXsNGA0gR0edXIx8Px5bU1xeh8DWQIN/TqGpMIEeOkUeY0qC+qAjsinET+wA WSMO6tJ2MR/QCP/nuQVh5a7sJxkzskVoaJfpvmxw6W9xpMFiLwijK3mlwctmw4lxp83x xANTaARdUvSRIsJKIBCc23jJ62r/JMIEigsDmh6ZlVkIXpAQeE/D7fMIQhvg3DxHj27q YQWG7r3QoY9AU+fdDWue4tym3FN5xeim0sIawnUcMYnZsppmvLH/2SpBM1vo0O934O/7 ZqojjX4bfJQ0vU/WCjrJoRkO3V/Vm64HbgLq3pHlx5YT4aknAsIotRE/52nK20Y1OdUw cKrA== MIME-Version: 1.0 X-Received: by 10.60.155.177 with SMTP id vx17mr296849oeb.9.1369789977105; Tue, 28 May 2013 18:12:57 -0700 (PDT) Received: by 10.76.169.68 with HTTP; Tue, 28 May 2013 18:12:57 -0700 (PDT) In-Reply-To: References: Date: Tue, 28 May 2013 18:12:57 -0700 Message-ID: From: Roberto Peon To: =?UTF-8?B?V2lsbGlhbSBDaGFuICjpmYjmmbrmmIwp?= Cc: James M Snell , HTTP Working Group , Patrick McManus Content-Type: multipart/alternative; boundary=089e010d812400990704ddd1157a Received-SPF: pass client-ip=209.85.219.49; envelope-from=grmocg@gmail.com; helo=mail-oa0-f49.google.com X-W3C-Hub-Spam-Status: No, score=-3.5 X-W3C-Hub-Spam-Report: AWL=-2.677, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, FREEMAIL_FROM=0.001, HTML_MESSAGE=0.001, RCVD_IN_DNSWL_LOW=-0.7, SPF_PASS=-0.001 X-W3C-Scan-Sig: lisa.w3.org 1UhUx8-0003mS-U4 4d4afdc8204e92a4e931d339d2f4a421 X-Original-To: ietf-http-wg@w3.org Subject: Re: Proposal - Reduce HTTP2 frame length from 16 to 12 bits Archived-At: Resent-From: ietf-http-wg@w3.org X-Mailing-List: archive/latest/18136 X-Loop: ietf-http-wg@w3.org Resent-Sender: ietf-http-wg-request@w3.org Precedence: list List-Id: List-Help: List-Post: List-Unsubscribe: --089e010d812400990704ddd1157a Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable I understand what you're saying. We're all agreeing that the max frame size should be limited. I'm simply suggesting 16k is what I think is the best small frame size. Patrick is suggesting 4k. I wouldn't object strenuously to 4k, but my experience and gut both say 16k is the better choice. -=3DR On Tue, May 28, 2013 at 4:25 PM, William Chan (=E9=99=88=E6=99=BA=E6=98=8C) wrote: > Just to be clear, I don't feel too strongly here. I do want to address a > point as I feel my previous point was lost. > > > On Tue, May 28, 2013 at 1:12 PM, Roberto Peon wrote: > >> responses inline >> >> >> On Tue, May 28, 2013 at 12:16 PM, William Chan (=E9=99=88=E6=99=BA=E6=98= =8C) < >> willchan@chromium.org> wrote: >> >>> On Tue, May 28, 2013 at 11:50 AM, James M Snell wrot= e: >>> >>>> On Tue, May 28, 2013 at 11:41 AM, Roberto Peon >>>> wrote: >>>> > As a reverse proxy, I've seen properties for which 4k writes/reads >>>> were too >>>> > small and induced latency increases. >>>> > >>>> >>>> I haven't played with this part too much yet but this is my general >>>> suspicion also. >>>> >>> >>> Can you guys clarify this in more detail? Specifically, where the >>> latency comes from. I have ideas, but I'd rather than an authoritative >>> explanation. >>> >> >> It always comes down to the cost of the context switches (i.e. syscalls) >> and the locking that must be done in the lower layers of the IO stack. >> > > Thanks for the clarification, I suspected it was the write()/read() cost, > which I assume is what you mean by syscall. > > >> >> >>> >>>> >>>> > Admittedly, frame size doesn't have to be the same as read/write >>>> size, but >>>> > it certainly does encourage that implementation (which is, I think, >>>> the >>>> > point of smaller max frame size that you proposed). >>>> >>> >>> You're right that it does encourage that implementation. Just like a >>> larger length encourages just naively breaking up frames into that max >>> frame size and thus hurt responsiveness. Which one is likelier to cause >>> worse overall "performance" (I know this is vague, since people care ab= out >>> different aspects of perf)? What we want to do is have the most reasona= ble >>> default behavior, with the ability for performant implementations to tu= ne >>> without unreasonable difficulty. I believe we're mostly focusing here o= n >>> optimizing the naive implementations, not the highly tuned implementati= ons. >>> >> >> Remember that I'm the one who proposed the smaller max frame size in the >> first place (now a fair while ago)? :) >> > > I don't believe I've said anything that would imply I forgot that :) > > >> My sweet-spot number was 16k, as I knew that I could saturate a 10G nic >> with 16k frames/writes and have enough CPU left over to do some actual >> work. The amount of overhead goes up more than linearly with the decreas= e >> in frame size thanks to contention, etc. >> > > I think you miss my point. Please correct me if I'm wrong, but I think > you're saying that for your server, 16k was the right choice for write()s= . > write() sizes don't need to be tied to actual frame size, but of course > that's what a naive implementation would do. And again, I think we should > pick a max frame size that results in reasonable behavior for naive > implementations/deployments. And I think the highly performant > implementations will want to write their code in a way that decouples fra= me > size from write() size, and will pick the optimal write() size given the > tradeoffs. > > >> >> >>> >>> >>>> > >>>> > I propose we keep the 16 bit frame size and instead allow the (now >>>> > negotiated setting of) max frame size to default to 12 bits worth, >>>> with that >>>> > going upwards out downwards when a settings frame arrives from the >>>> other >>>> > side indicating it's max receive size. HK >>>> > >>>> >>>> Honestly, I'd prefer to do away with frame size negotiation altogether >>>> because of the potential for path mtu style issues. Keeping the 16-bit >>>> size for now with strong encouragement (SHOULD, perhaps?) for keeping >>>> sizes around 12-bit lengths for the most common cases seems like the >>>> right approach. >>>> >>>> -- James >>>> >>> >> Unlike TCP/IP, max frame size is a point-to-point thing, as the primitiv= e >> we mux is streams, not frames. Frames are the way we accomplish the muxi= ng. >> Why would there be any path MTU like thing? >> >> -=3DR >> >> >>> >>>> > This would give the best chance that the code would be written in >>>> such a way >>>> > as to adapt with the times as they change. >>>> > -=3DR >>>> > >>>> > On May 28, 2013 10:01 AM, "William Chan (=E9=99=88=E6=99=BA=E6=98=8C= )" >>> > >>>> > wrote: >>>> >> >>>> >> Can you clarify what you mean by a documented performance metric fo= r >>>> >> non-browser use cases? I don't think Patrick said anything browser >>>> specific. >>>> >> He provided some serialization latency numbers and noted that they >>>> are high >>>> >> enough to impact responsiveness. And then he provided numbers on >>>> overhead. >>>> >> >>>> >> I, for one, find the responsiveness argument compelling for >>>> browsers. I'm >>>> >> not completely sure 0.2% is low enough overhead for everyone, but I >>>> wouldn't >>>> >> complain about it. And in absence of complaints, I guess I'd suppor= t >>>> moving >>>> >> forward with only 12 bits for length. >>>> >> >>>> >> >>>> >> On Tue, May 28, 2013 at 9:22 AM, James M Snell >>>> wrote: >>>> >>> >>>> >>> Currently, my only challenge with this is that, so far, we have no= t >>>> >>> seen any documented performance metrics for non-browser based uses= . >>>> >>> .That said, I don't really have the time currently to put together= a >>>> >>> comprehensive set of such metrics so it wouldn't be polite of me t= o >>>> >>> insist on them ;-) ... perhaps for now we ought to keep the 16-bit >>>> >>> size but include a recommendation about not exceeding 12-bits, the= n >>>> >>> see what more implementation experience does for us. >>>> >>> >>>> >>> On Tue, May 28, 2013 at 7:20 AM, Patrick McManus < >>>> mcmanus@ducksong.com> >>>> >>> wrote: >>>> >>> > Hi All, >>>> >>> > >>>> >>> > I've been looking at a lot of spdy frames lately, and I've >>>> noticed what >>>> >>> > I >>>> >>> > consider a common implementation problem that I think a good >>>> http/2 >>>> >>> > spec >>>> >>> > could help with. I'm commonly seeing frames large enough to >>>> interfere >>>> >>> > with >>>> >>> > effective prioritization. I've seen this from at least 3 differe= nt >>>> >>> > servers. >>>> >>> > >>>> >>> > The HTTP/2 draft has a max frame size of 16 bits, which is a hug= e >>>> >>> > improvement from spdy's 24. I propose we reduce it further to 12= . >>>> (i.e. >>>> >>> > 4096 >>>> >>> > bytes). >>>> >>> > >>>> >>> > The muxxed approach of multiple streams onto one connection done >>>> in >>>> >>> > HTTP/2 >>>> >>> > has great advantages, but the one downside of it is that it >>>> creates >>>> >>> > head of >>>> >>> > line blocking problems between those streams dictated by frame >>>> >>> > granularity. >>>> >>> > With small frames this is pretty manageable, with extremely larg= e >>>> ones >>>> >>> > we've >>>> >>> > recreated the same head of line problems that HTTP/1 pipelines >>>> have. >>>> >>> > The >>>> >>> > server needs to be able to respond quickly to higher priority >>>> events >>>> >>> > (including cancellations) and once it has written a frame header >>>> to the >>>> >>> > wire >>>> >>> > it is committed to the entire frame for how ever long it takes t= o >>>> >>> > serialize >>>> >>> > it. IMO the shorter that time, the better. >>>> >>> > >>>> >>> > Our spec can help implementations do the right thing here by >>>> limiting >>>> >>> > the >>>> >>> > max frame size to 12 bits. >>>> >>> > >>>> >>> > It takes 500msec to serialize 64KB at 1Mbit/sec... 125msec at >>>> >>> > 4Mbit/sec. >>>> >>> > Those are some pretty notable task-switch times. Dropping the >>>> frame to >>>> >>> > 4096 >>>> >>> > cuts them to 32msec and 8 msec.. that's much more responsive, at >>>> the >>>> >>> > cost of >>>> >>> > 120 extra bytes of transfer (< 1msec at 1Mbit/sec). >>>> >>> > >>>> >>> > In general - the smaller the better as long as the overhead >>>> doesn't get >>>> >>> > to >>>> >>> > be too large. At 8 in 4096 (~.2%) I think that's acceptable. Its >>>> >>> > roughly the >>>> >>> > same overhead as a VLAN tag. >>>> >>> > >>>> >>> > Obviously this makes a continuation bit for control frames >>>> absolutely >>>> >>> > mandatory, but I think we're already in that spot with 16 bit >>>> frame >>>> >>> > lengths. >>>> >>> > >>>> >>> > -Patrick >>>> >>> > >>>> >>> > >>>> >>> >>>> >> >>>> > >>>> >>> >>> >> > --089e010d812400990704ddd1157a Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
I understand what you're saying. We're all agreein= g that the max frame size should be limited. I'm simply suggesting 16k = is what I think is the best small frame size. Patrick is suggesting 4k. I wouldn't object strenuously to 4k, but my experience and gut both say= 16k is the better choice.

-=3DR
=


On Tue,= May 28, 2013 at 4:25 PM, William Chan (=E9=99=88=E6=99=BA=E6=98=8C) <= willchan@chromium.org> wrote:
Just to be clear, I don'= ;t feel too strongly here. I do want to address a point as I feel my previo= us point was lost.


On Tue, May 28, 2013 a= t 1:12 PM, Roberto Peon <grmocg@gmail.com> wrote:
res= ponses inline


On Tue, May 28, 2013 at 12:16 PM, William Chan (=E9=99=88=E6=99=BA=E6=98=8C= ) <willchan@chromium.org> wrote:
On Tue, May 28, 2013 a= t 11:50 AM, James M Snell <jasnell@gmail.com> wrote:
On Tue, May 28, 2013 at 11:41 AM, Rober= to Peon <grmocg@gm= ail.com> wrote:
> As a reverse proxy, I've seen properties for which 4k writes/reads= were too
> small and induced latency increases.
>

I haven't played with this part too much yet but this is my gener= al
suspicion also.

Can you guys clar= ify this in more detail? Specifically, where the latency comes from. I have= ideas, but I'd rather than an authoritative explanation.

It always comes down to = the cost of the context switches (i.e. syscalls) and the locking that must = be done in the lower layers of the IO stack.

Thanks for the clarification, I suspected it was = the write()/read() cost, which I assume is what you mean by syscall.
<= div class=3D"im">
=C2=A0

=C2=A0

> Admittedly, frame size doesn't have to be the same as read/write s= ize, but
> it certainly does encourage that implementation (which is, I think, th= e
> point of smaller max frame size that you proposed).

You're right that it does encourage that = implementation. Just like a larger length encourages just naively breaking = up frames into that max frame size and thus hurt responsiveness. Which one = is likelier to cause worse overall "performance" (I know this is = vague, since people care about different aspects of perf)? What we want to = do is have the most reasonable default behavior, with the ability for perfo= rmant implementations to tune without unreasonable difficulty. I believe we= 're mostly focusing here on optimizing the naive implementations, not t= he highly tuned implementations.

Remember that I= 9;m the one who proposed the smaller max frame size in the first place (now= a fair while ago)? :)

I don't believe I've said anything that would imply I fo= rgot that :)
=C2=A0
My sweet-spot number was 16k, as I knew tha= t I could saturate a 10G nic with 16k frames/writes and have enough CPU lef= t over to do some actual work. The amount of overhead goes up more than lin= early with the decrease in frame size thanks to contention, etc.

I think you miss m= y point. Please correct me if I'm wrong, but I think you're saying = that for your server, 16k was the right choice for write()s. write() sizes = don't need to be tied to actual frame size, but of course that's wh= at a naive implementation would do. And again, I think we should pick a max= frame size that results in reasonable behavior for naive implementations/d= eployments. And I think the highly performant implementations will want to = write their code in a way that decouples frame size from write() size, and = will pick the optimal write() size given the tradeoffs.
=C2=A0
=C2=A0
=C2=A0
>
> I propose we keep the 16 bit frame size and instead allow the (now
> negotiated setting of) max frame size to default to 12 bits worth, wit= h that
> going upwards out downwards when a settings frame arrives from the oth= er
> side indicating it's max receive size. HK
>

Honestly, I'd prefer to do away with frame size negotiation altog= ether
because of the potential for path mtu style issues. Keeping the 16-bit
size for now with strong encouragement (SHOULD, perhaps?) for keeping
sizes around 12-bit lengths for the most common cases =C2=A0seems like the<= br> right approach.

-- James

Unlike TCP/IP, max frame size is a point-t= o-point thing, as the primitive we mux is streams, not frames. Frames are t= he way we accomplish the muxing.
Why would there be any path MTU like thing?

-=3DR
=C2=A0=

> This would give the best chance that the code would be written in such= a way
> as to adapt with the times as they change.
> -=3DR
>
> On May 28, 2013 10:01 AM, "William Chan (=E9=99=88=E6=99=BA=E6=98= =8C)" <w= illchan@chromium.org>
> wrote:
>>
>> Can you clarify what you mean by a documented performance metric f= or
>> non-browser use cases? I don't think Patrick said anything bro= wser specific.
>> He provided some serialization latency numbers and noted that they= are high
>> enough to impact responsiveness. And then he provided numbers on o= verhead.
>>
>> I, for one, find the responsiveness argument compelling for browse= rs. I'm
>> not completely sure 0.2% is low enough overhead for everyone, but = I wouldn't
>> complain about it. And in absence of complaints, I guess I'd s= upport moving
>> forward with only 12 bits for length.
>>
>>
>> On Tue, May 28, 2013 at 9:22 AM, James M Snell <jasnell@gmail.com> wrote: >>>
>>> Currently, my only challenge with this is that, so far, we hav= e not
>>> seen any documented performance metrics for non-browser based = uses.
>>> .That said, I don't really have the time currently to put = together a
>>> comprehensive set of such metrics so it wouldn't be polite= of me to
>>> insist on them ;-) ... perhaps for now we ought to keep the 16= -bit
>>> size but include a recommendation about not exceeding 12-bits,= then
>>> see what more implementation experience does for us.
>>>
>>> On Tue, May 28, 2013 at 7:20 AM, Patrick McManus <mcmanus@ducksong.com= >
>>> wrote:
>>> > Hi All,
>>> >
>>> > I've been looking at a lot of spdy frames lately, and= I've noticed what
>>> > I
>>> > consider a common implementation problem that I think a g= ood http/2
>>> > spec
>>> > could help with. I'm commonly seeing frames large eno= ugh to interfere
>>> > with
>>> > effective prioritization. I've seen this from at leas= t 3 different
>>> > servers.
>>> >
>>> > The HTTP/2 draft has a max frame size of 16 bits, which i= s a huge
>>> > improvement from spdy's 24. I propose we reduce it fu= rther to 12. (i.e.
>>> > 4096
>>> > bytes).
>>> >
>>> > The muxxed approach of multiple streams onto one connecti= on done in
>>> > HTTP/2
>>> > has great advantages, but the one downside of it is that = it creates
>>> > head of
>>> > line blocking problems between those streams dictated by = frame
>>> > granularity.
>>> > With small frames this is pretty manageable, with extreme= ly large ones
>>> > we've
>>> > recreated the same head of line problems that HTTP/1 pipe= lines have.
>>> > The
>>> > server needs to =C2=A0be able to respond quickly to highe= r priority events
>>> > (including cancellations) and once it has written a frame= header to the
>>> > wire
>>> > it is committed to the entire frame for how ever long it = takes to
>>> > serialize
>>> > it. IMO the shorter that time, the better.
>>> >
>>> > Our spec can help implementations do the right thing here= by limiting
>>> > the
>>> > max frame size to 12 bits.
>>> >
>>> > It takes 500msec to serialize 64KB at 1Mbit/sec... 125mse= c at
>>> > 4Mbit/sec.
>>> > Those are some pretty notable task-switch times. Dropping= the frame to
>>> > 4096
>>> > cuts them to 32msec and 8 msec.. that's much more res= ponsive, at the
>>> > cost of
>>> > 120 extra bytes of transfer (< 1msec at 1Mbit/sec). >>> >
>>> > In general - the smaller the better as long as the overhe= ad doesn't get
>>> > to
>>> > be too large. At 8 in 4096 (~.2%) I think that's acce= ptable. Its
>>> > roughly the
>>> > same overhead as a VLAN tag.
>>> >
>>> > Obviously this makes a continuation bit for control frame= s absolutely
>>> > mandatory, but I think we're already in that spot wit= h 16 bit frame
>>> > lengths.
>>> >
>>> > -Patrick
>>> >
>>> >
>>>
>>
>




--089e010d812400990704ddd1157a--