Joachim, > On 15 Dec 2016, at 11:43, jtuc...@objektfabrik.de wrote: > > Victor, > > Am 14.12.16 um 19:23 schrieb Vitor Medina Cruz: >> If I tell you that my current estimate is that a Smalltalk image with >> Seaside will not be able to handle more than 20 concurrent users, in many >> cases even less. >> >> Seriously? That is kinda a low number, I would expect more for each image. >> Certainly it depends much on many things, but it is certainly very low for a >> rough estimate, why you say that? > > seriously, I think 20 is very optimistic for several reasons. > > One, you want to be fast and responsive for every single user, so there is > absolutely no point in going too close to any limit. It's easy to lose users > by providing bad experience. > > Second, in a CRUD Application, you mostly work a lot with DB queries. And you > connect to all kinds of stuff and do I/O. Some of these things simply block > the VM. Even if that is only for 0.3 seconds, you postpone processing for > each "unaffected" user by these 0.3 seconds, so this adds to significant > delays in response time. And if you do some heavy db operations, 0.3 seconds > is not a terribly bad estimate. Add to that the materialization and stuff > within the Smalltalk image. > > Seaside adapters usually start off green threads for each request. But there > are things that need to be serialized (like in a critical Block). So in > reality, users block each other way more often than you'd like. > > So if you asked me to give a more realistic estimation, I'd correct myself > down to a number between 5 and probably a maximum of 10 users. Everything > else means you must use all those fancy tricks and tools people mention in > this thread. > So what you absolutely need to do is start with an estimate of 5 concurrent > users per image and look for ways to distribute work among servers/images so > that these blocking situations are down to a minimum. If you find your > software works much better, congratulate yourself and stack up new machines > more slowly than initially estimated. > > > Before you turn around and say: Smalltalk is unsuitable for the web, let's > take a brief look at what concurrent users really means. Concurrent users are > users that request some processing from the server at they very same time > (maybe within an interval of 200-400msec). This is not the same as 5 people > being currently logged on to the server and requesting something sometimes. 5 > concurrent users can be 20, 50, 100 users who are logged in at the same time. > > Then there is this sad "share all vs. share nothing" argument. In Seaside you > keep all your objects alive (read from db and materialized) between web > requests. IN share nothing, you read everything back from disc/db whenever a > request comes in. This also takes time and ressources (and pssibly blocks the > server for the blink of an eye or two). You exchange RAM with CPU cycles and > I/O. It is extremely hard to predict what works better, and I guess nobody > ever made A/B tests. It's all just theoretical bla bla and guesses of what > definitely must be better in one's world. > > Why do I come up with this share everything stuff? Because it usually means > that each user that is logged on holds onto a load of objects on the server > side (session storage), like their user account, shopping card, settings, > last purchases, account information and whatnot. That's easily a list of a > few thousand objects (and be it only Proxies) that take up space and want to > be inspected by the garbage collector. So each connected user not only needs > CPU cycles whenever they send a request to the server, but also uses RAM. In > our case, this can easily be 5-10 MB of objects per user. Add to that the > shadow copies that your persistence mechanism needs for undo and stuff, and > all the data Seaside needs for Continuations etc, and each logged on users > needs 15, 20 or more MB of object space. Connect ten users and you have > 150-200 MB. That is not a problem per se, but also means there is some hard > limit, especially in a 32 bit world. You don't want your server to slow down > because it cannot allocate new memory or can't find contiguous slots for > stuff and GCs all the time. > > To sum up, I think the number of influencing factors is way too high to > really give a good estimate. Our experience (based on our mix of computation > and I/O) says that 5 concurrent users per image is doable without negative > impact on other users. Some operations take so much time that you really need > to move them out of the front-facing image and distribute work to backend > servers. More than 5 is probably possible but chances are that there are > operations that will affect all users and with every additional user there is > a growing chance that you have 2 or more requesting the yery same operation > within a very short interval. This will make things worse and worse. > > So I trust in you guys having lots of cool tools around and knowing loads of > tricks to wrench out much more power of a single Smalltalk image, but you > also need to take a look at your productivity and speed in creating new > features and fixing bugs. Sometimes throwing hardware at a problem like > growth and starting with a clever architecture to scale on multiple layers is > just the perfect thing to do. To me, handling 7 instead of 5 concurrent users > is not such a big win as long as we are not in a posotion where we have so > many users that this really matters. For sites like Amazon, Google, Facebook > etc. saving 40% in server cost by optimizing the software (investing a few > man years) is significant. I hope we'll soon change our mind about this > question ;-) > > So load balancing and services outsourced to backend servers are key to > scalability. This, btw, is not smalltalk specific (some people seem to think > you won't get these problems in Java or Ruby because they are made for the > web...). > > Joachim
Everything you say, all your considerations, especially the last paragraph is/are correct and I agree. But some people will only remember the very low number you seem to be suggesting (which is more of a worse case scenario, with Seaside+blocking/slow connections to back end systems). One the other hand, plain HTTP access to a Pharo image can be quite fast. Here is quick & dirty benchmark I just did on one of our modern/big machines (inside an LXD container, light load) using a single stock image on Linux. $ pharo Pharo.image printVersion [version] 4.0 #40626 $ pharo Pharo.image eval 'ZnServer startDefaultOn: 1701. 1 hour wait' & $ ab -k -c 8 -n 10240 http://127.0.0.1:1701/bytes/32 This is ApacheBench, Version 2.3 <$Revision: 1638069 $> Copyright 1996 Adam Twiss, Zeus Technology Ltd, http://www.zeustech.net/ Licensed to The Apache Software Foundation, http://www.apache.org/ Benchmarking 127.0.0.1 (be patient) Completed 1024 requests Completed 2048 requests Completed 3072 requests Completed 4096 requests Completed 5120 requests Completed 6144 requests Completed 7168 requests Completed 8192 requests Completed 9216 requests Completed 10240 requests Finished 10240 requests Server Software: Zinc Server Hostname: 127.0.0.1 Server Port: 1701 Document Path: /bytes/32 Document Length: 32 bytes Concurrency Level: 8 Time taken for tests: 1.945 seconds Complete requests: 10240 Failed requests: 0 Keep-Alive requests: 10240 Total transferred: 2109440 bytes HTML transferred: 327680 bytes Requests per second: 5265.17 [#/sec] (mean) Time per request: 1.519 [ms] (mean) Time per request: 0.190 [ms] (mean, across all concurrent requests) Transfer rate: 1059.20 [Kbytes/sec] received Connection Times (ms) min mean[+/-sd] median max Connect: 0 0 0.0 0 2 Processing: 0 2 8.0 2 309 Waiting: 0 1 8.0 1 309 Total: 0 2 8.0 2 309 Percentage of the requests served within a certain time (ms) 50% 2 66% 2 75% 2 80% 2 90% 2 95% 3 98% 3 99% 3 100% 309 (longest request) More than 5K req/s (10K requests, 8 concurrent clients). Granted, this is only for just 32 bytes payload and the loopback network interface. But this is the other end of the interval, the maximum speed. A more realistic payload (7K HTML) gives the following: $ ab -k -c 8 -n 10240 http://127.0.0.1:1701/dw-bench This is ApacheBench, Version 2.3 <$Revision: 1638069 $> Copyright 1996 Adam Twiss, Zeus Technology Ltd, http://www.zeustech.net/ Licensed to The Apache Software Foundation, http://www.apache.org/ Benchmarking 127.0.0.1 (be patient) Completed 1024 requests Completed 2048 requests Completed 3072 requests Completed 4096 requests Completed 5120 requests Completed 6144 requests Completed 7168 requests Completed 8192 requests Completed 9216 requests Completed 10240 requests Finished 10240 requests Server Software: Zinc Server Hostname: 127.0.0.1 Server Port: 1701 Document Path: /dw-bench Document Length: 7734 bytes Concurrency Level: 8 Time taken for tests: 7.874 seconds Complete requests: 10240 Failed requests: 0 Keep-Alive requests: 10240 Total transferred: 80988160 bytes HTML transferred: 79196160 bytes Requests per second: 1300.46 [#/sec] (mean) Time per request: 6.152 [ms] (mean) Time per request: 0.769 [ms] (mean, across all concurrent requests) Transfer rate: 10044.25 [Kbytes/sec] received Connection Times (ms) min mean[+/-sd] median max Connect: 0 0 0.0 0 0 Processing: 1 6 183.4 1 7874 Waiting: 1 6 183.4 1 7874 Total: 1 6 183.4 1 7874 Percentage of the requests served within a certain time (ms) 50% 1 66% 1 75% 1 80% 1 90% 1 95% 1 98% 1 99% 1 100% 7874 (longest request) That is more than 1K req/s. In both cases we are talking about sub 1ms req/resp cycles ! I think all commercial users of Pharo today know what is possible and what needs to be done to achieve their goals. Pure speed might not be the main consideration, ease/speed/joy of development and just being capable of solving complex problems and offering compelling solutions to end users is probably more important. Sven
