Thank you once again for your swift response!
On Friday, March 20, 2015 at 2:33:59 PM UTC+8, Gustaf Neumann wrote:
>
> Am 20.03.15 um 05:47 schrieb Sep Ng:
>
> Hi Gustaf! Thank you for the informative response!
>
> I've been thinking of moving to NaviServer but I don't know enough about
> the transition to make that call yet. Right now, we're on aolserver and
> so, I'm trying to see what I can do on this platform. I do not understand
> why the delivery doesn't work on https out of the box and requires a
> reverse proxy.
>
> bgdelivery takes the socket (file descriptor) of the current connection,
> but it has no knowledge about SSL. When it hands the file descriptor to the
> background delivery thread, this can write back to the client just using
> plain tcl i/o. So, background delivery can certainly write to the
> file-descriptor, but that won't be accepted by the client trying to decrypt
> the channel.
>
I think that is now making more sense now. Thanks. I may have to look
into this as well.
>
> I suspect the varied client connection is part of the problem and them
> sitting on the connection threads is hurting us.
>
> what is hurting you?
>
We have instances where we'd get a high number of concurrent users that the
requests are getting queued, but when I look at the logs, there's a lot of
static files being served for each login page, let alone other pages being
served in aolserver. So, I'm theorizing that being able to get those
static file requests pushed into a single thread and free up the connection
threads would help in scalability.
> However, we do not serve big files on our server so this has me
> wondering about the benefits of this change.
>
> whatever big means. connections can "hang" also when writing a few KBs.
>
Interesting.
>
> I'm not certain if aolserver has any facilities for asynchronous file
> writing and spooling.
>
> the writer threads are an extension of naviserver over aolserver
>
> It seems that I will have to build everything by hand. I had hoped
> that simply transferring the thread and having it ns_returnfile would be
> enough to get a simple form of background delivery going but it doesn't
> look like that's the case.
>
> if your site requires https, one cant use bgdelivery without a reverse
> proxy.
> otherwise, everything is pre-packaged.
>
> Oh. We don't use OpenACS as everything here is custom built by me and
others before me. So, it's looking like I'm going to have to roll up my
sleeves and get to work.
By the way, I've seen in previous posts of yours that the you did switch
from aolserver to naviserver. How big was the change? What things did you
have to re-write/port to get them running in naviserver?
Regards.
> -g
>
>
> Regards.
>
>
> On Friday, March 20, 2015 at 12:03:52 PM UTC+8, Gustaf Neumann wrote:
>>
>> Dear Sep,
>>
>> The question whether it is worth to use asynchronous delivery boils
>> down to a question of usage pattern and desired scalability.
>> The general problem with serving (large) resources via
>> classical aolserver is that a connection thread is unable
>> to handle other threads for the time span of the delivery.
>> It is important to understand that the time span of the delivery is
>> mostly
>> determined by the client. A client with little processing power
>> connection
>> over e.g. a mobile phone can block a connection quite a long time. A
>> special instance of this is the slow-read attack [2], which is
>> a special denial-of-service attack.
>>
>> To serve e.g. 60 concurrent files one would require 60
>> connection threads. Note that this can happen quite soon when
>> serving content with several included resources (images, css, js)
>> the first time to a client. When the server runs out of connection
>> threads, the requests are queued, which means that the
>> the user-perceived runtime of a request is actually queueing
>> time plus execution time.
>>
>> Background delivery (as described in [2]) is fully integrated in OpenACS
>> addresses the problem by delegating output spooling (file deliveries)
>> to a single thread, which can deliver easily several 100 concurrent
>> downloads by using Tcl's asynchronous I/O operations. Note that
>> this works not only for static resources, but as well dynamic
>> requests (e.g. generating long HTML pages from e.g. a database).
>> We used this approach with very good success since 2006
>> in large OpenACS installations (with e.g. 2000 simultaneous
>> active users; "simultaneous active" means here users who
>> requested pages within a time interval of 5 secs).
>>
>> In OpenACS, one can use simply ad_returnfile_background [3]
>> instead of ad_returnfile to make use of background delivery.
>>
>> The limitations of background delivery are that (a) it just works for
>> plain http, and (b) that it works for at most 1024 concurrently open
>> file handles. We addressed (a) by using a reverse proxy in front
>> of the server, which delivers the files from the backend via https.
>> The limitation (b) is harder, since it depends on Tcl's usage of the
>> select()
>> system call, which allows to wait for events for max. 1024 file
>> descriptors. Above this limit, it simply crashes. Lifting this limit
>> in systems like Linux is possible, but requires a privately compiled
>> libc and linux kernel. You might think, 1024 this is much more
>> one needs, but we were actually running close to this limit for
>> lecture casting (video streaming of university lectures).
>>
>> A better approach is to use NaviServer.'s c-level support.
>> NaviServer provides lightweight c-implemented
>> writer-threads using asynchronous I/O similar to
>> bg-delivery, but not using select(). The writer threads
>> works seemless with http and https. As with bgdelivery, a single
>> writer thread can serve a multitude of concurrent deliveries.
>> When several writer threads are defined, the load is split up
>> between these. NaviServer can also serve streaming
>> HTML (multiple ns_write commands) via writer threads.
>> It also support static and dynamic gzip deliveries see e.g. [3]
>>
>> When one uses OpenACS with NaviServer it will automatically use
>> writer-threads when configured. In reference [4] on can see the
>> difference in response time (actually the time duration spent
>> in connection threads) in NaviServer. OpenACS.org runs
>> on NaviServer since Sep 2014. A more detailed discussion
>> of these properties is in [5], all of this is part of NaviServer 4.99.6.
>>
>> sorry for the longish reply,
>> -g
>>
>> [1]
>>
>> http://openacs.org/xowiki/Boost_your_application_performance_to_serve_large_files!
>>
>>
>> [2]
>> http://en.wikipedia.org/wiki/Denial-of-service_attack#Slow_Read_attack
>> [3]
>>
>> http://openacs.org/api-doc/proc-view?proc=ad_returnfile_background&source_p=1
>>
>> [3] http://www.qcode.co.uk/post/121
>> [4] http://openacs.org/forums/message-view?message_id=4111406
>> [5]
>>
>> https://next-scripting.org/xowiki/docs/misc/naviserver-connthreadqueue/index1
>>
>>
>> Am 19.03.15 um 07:09 schrieb Sep Ng:
>> > Hi all,
>> >
>> > I've been reading up on aolserver background delivery tricks on
>> > OpenACS and I've seen that the patches for the static TCL channel is
>> > already in 4.5.1. In the spirit of improving server performance, I've
>> > been wondering if such facility is worth building on the custom app to
>> > increase concurrency and scalability.
>> >
>> > Most of the time, our aolserver also has to handle incoming requests
>> > for multiple jpeg, javascript libraries, and a lot of other things.
>> > Freeing up the connection thread sounds very useful in improving the
>> > server scalability so I wanted a little bit of help on getting this to
>> > work.
>> >
>> > It's been hard trying to wrap my head around using ns_conn channel and
>> > what I can actually do with this static TCL thread. It seems that I
>> > should be redefining ns_returnfile to use background delivery. Could
>> > I use it to push a TCL proc that generates given the parameters, the
>> > dynamic page to this TCL channel to free up my connections?
>> >
>> > Sep
>>
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>>
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>
>
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