Working as Expected (Jul 13 2009)


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Message 919213 - Posted: 19 Jul 2009, 2:39:46 UTC - in response to Message 919185.
Last modified: 19 Jul 2009, 2:54:07 UTC

Martin,
I agree with a lot of what you're saying. This problem is actually very simple.

Demand for WU is greater than the WU creation rate. If I'm doing my math correctly, WU creation rate peaks around 8.2MB/sec (corresponds to 23 WU/sec). Demand is already exceeding this and would probably be higher if they had the bandwidth.

Short of finding a way to DOUBLE the WU creation rate, the only option is to add latency to the download rate. The easiest way to do this would be to cap download bandwidth at the router. Is there traffic shaping imposed? If not, I would be shocked, as this is the quickest and easiest way to help the situation (assuming the router(s) in place have this capability).

It makes no sense to flood the clients with WU because it makes the database (results in the field) grow to an unmanageable size. So, the only quick solution for now is to flow-control the download speeds. By having slower, more reliable downloads/transactions, there will be less retries/resends. It should also give the splitters a little breathing room to build a queue (during the slow times of day).

Edit: OK, I see the splitter WU creation rate at 39 WU/sec..corresponding to 14.1 MB/sec of WU. Perhaps there is some I/O contention because less people are downloading querying this late at night. Still, my recommendation for traffic shaping (or changing its parameters) still stands.

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Message 919222 - Posted: 19 Jul 2009, 3:32:25 UTC - in response to Message 919213.
Last modified: 19 Jul 2009, 3:34:39 UTC

Edit: OK, I see the splitter WU creation rate at 39 WU/sec..corresponding to 14.1 MB/sec of WU. Perhaps there is some I/O contention because less people are downloading querying this late at night.

The splitters have on occasion pumped out as much as 50 MultiBeam Work Units per second, which is way more than required. Generally around 15-18/s is enough to meet demand. Any more than that builds up the Ready to Send buffer. When there are a lot of shorties, around 20-25/s is enough to meet demand, any more than that builds up the buffer.
With AP the present demand is a bit more difficult to work out, but it would appear that once caches are full, 1-2 per second is more than enough to build up a ready to send buffer.
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Message 919229 - Posted: 19 Jul 2009, 3:47:34 UTC - in response to Message 919185.

As a bodge-fix, just simply limit the WU supply to limit the download traffic to less than 80Mbit/s?

At this point, the problem isn't the newly assigned work, but work already downloaded and work that has been completed and not yet uploaded.

Stopping work unit production completely would stop uploads, but the download link would still be saturated until they all get through.

Crossed wires on the directions?...

Note that http://fragment1.berkeley.edu/newcricket/grapher.cgi?target=%2Frouter-interfaces%2Finr-250%2Fgigabitethernet2_3;ranges=d;view=Octets shows the view wrt the router at "the bottom of the hill looking up". The saturated direction is downloads: Berkeley servers -> clients around the world.

In whatever way, the rate at which new WUs are made available for download shouldn't exceed the link capacity including a good margin for bursts. Indeed, the present overload won't clear until the presently assigned WUs have cleared or their release rate is controlled. Or unless packet level traffic management is imposed...

The uploads (client WU results -> Berkeley servers) have plenty of spare bandwidth to freely upload IF the upload tcp connections had a guaranteed success for return data packets to get through the downlink. There is a recent demonstration of the effect mentioned here and also here.

Whatever is done, wherever, and at what level, the link in BOTH directions must be kept at something like 89Mbit/s or less for 'smooth' operation to gain MAXIMUM transfer rates.

Although the link shows 90+ Mbit/s downlink, with all the repeated resends due to dropped packets, there's going to be very much less than 90Mbit/s of useful data making it through. That is, the effective bandwidth will be very poor whilst saturated.

The source problem is in allowing an unlimited flood of data into a very finite internet connection. Infinite into finite doesn't work...

All of which I'm sure must be obvious.

(Note that data link "policing" is highly wasteful of data bandwidth. Sure, tcp will mop up the mess, but at a high cost of greatly wasted bandwidth...)

Happy crunchin',
Martin

The source of the problem is allowing an unlimited flood of data into or out of a very finite internet connection.

So, yeah, maybe a little crossed up on directions, but all the same problem.

Data coming out of SETI right now is a mix of downloaded work units, responses from the upload server(s) and scheduler responses, plus the usual TCP overhead.

Heavy outbound traffic will affect inbound traffic.

I know I'm concentrating on the upload problems maybe a little much, but it's all of a thing.

What if the feeder was limited to about 17 megabytes at each two minute "feed"?

... or, take the mix of multibeam and astropulse (97:3), add up the normal sizes for each WU type, and run the feeder so that it feeds 8.5 megabytes per second (about 3/4ths of the bandwidth), then tune from there.

I've seen someone comment on the relative file sizes, so doing the math should be easy.
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Message 919343 - Posted: 19 Jul 2009, 13:47:14 UTC

Well it's Sunday afternoon here in the UK, and SETI is being very, very very slow.
In deed its so slow as to be almost dead. The number of WU completed at this end, and not delivered to Berkeley is increasing by more than one an hour, at the same time I'm not getting any fresh data to chew on. So things are far from right.
Sitting here, and not knowing the full story it looks like the size of WU is too small for modern processors - my slowest (oldest) is chugging along with two "sets" of WU speeds, one taking about 30 hours, and the other about 8 hours, while the two fastest are running at about 10% of that time. Given that the disparity in speed I would have thought it better to target the larger (longer execution time) WU at the faster processors, but this is not the case - the fast processors are getting a disproportionate number of the "smaller" (faster to execute) WU, so are clogging Berkeley with many finished WU, and naturally many requests for new WU.
Perhaps a slightly different approach is needed in the distribution of WU - assuming that the WU distribution understands the performance of the requesting processor it should be possible to ensure that there is a sort of match between the processor and the "size" of the delivered WU. After all it does understand not to send Astopulse WU to my slowest processors - it made the mistake once, and eventually baled out long beyond the expiry date of the WU.
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Message 919359 - Posted: 19 Jul 2009, 14:17:09 UTC

Continuing the discussion of limiting work assignment to avoid download saturation, I agree it could at least improve the situation.

S@H Enhanced WUs average about 375350 bytes, converting to bits and allowing 2.7% packet overhead gives about 3.084 MBits. Similarly, AP WUs are about 68.95 MBits.

One channel of a 50.2 GB file gives 62 groups of 256 Enhanced WUs (15872) and 400 AP WUs, a 39.68 ratio. Shorter files, channels which end in error, and so forth make some variation in the ratio, but delivery of work should be around 40 Enhanced tasks for each AP task to avoid either type getting ahead.

When there were only Enhanced and AP_v5, the Feeder weights were set to 97 and 3. Addition of AP_v505 probably changed that to 96, 1, and 3. My suggestion as part of limiting work assignment is 80, 18, and 2. The 18 slots for AP_v5 would almost always be empty since reissue of old work is rare by this time. The 80:2 ratio for Enhanced:AP_v505 should be close to ideal.

80*3.084 MBits + 2*68.95 MBits gives 384.62 MBits of equivalent download each time the Feeder runs. If the default 5 second sleep time for the Feeder were used, that would average to slightly under 77 MBits/second. With some packet resends even under the best of circumstances, that's close to the 80 MBits/second Martin suggests.

The boinc_sleep() used for the feeder sleep time allows using fractional seconds, so the default 5 seconds could be adjusted to try for increased delivery. 4.5 seconds would boost from 77 MBits/second to 85.5 MBits/second for instance.

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Message 919374 - Posted: 19 Jul 2009, 15:04:03 UTC

Thanks Joe,
I start to see what the process is, but I'm still concerned that the balance of the distribution of WU could be improved to make better use of the target computers.
If I understand the distribution process it goes something like this:
A call is made by a computer out in the field for some more data because the local cache has been consumed below its trigger level.
The computer identifies itself to the WU distribution server.
A check is made by the distribution server to see what sort of WU the requesting computer can accept.
If there is a suitable WU available then it is sent
Currently the only major filter is on AP WU, which appear to be blocked to computers that are too slow.

OK?

So what about enhancing the pre-filtering of Enhanced WU?
Use two classes for the WU "fast to compute" and "slow to compute"
Tag computers with a "performance class" - again only two classes needed "slow computer" "fast computer".
WU are delivered then delivered on a simple set of rules:
"Fast" computer preferentially gets "slow to compute" WU
"Slow" computer preferentially gets "fast to compute" WU

The net result should be a smoothing out of the time taken for any given WU to be processed, thus giving a smoother demand on the distribution and recovery servers.
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Message 919383 - Posted: 19 Jul 2009, 15:29:25 UTC - in response to Message 919374.
Last modified: 19 Jul 2009, 15:31:17 UTC

So what about enhancing the pre-filtering of Enhanced WU?
Use two classes for the WU "fast to compute" and "slow to compute"
Tag computers with a "performance class" - again only two classes needed "slow computer" "fast computer".
WU are delivered then delivered on a simple set of rules:
"Fast" computer preferentially gets "slow to compute" WU
"Slow" computer preferentially gets "fast to compute" WU

The net result should be a smoothing out of the time taken for any given WU to be processed, thus giving a smoother demand on the distribution and recovery servers.


The problem is in defining "slow" and "fast" computers. Where do you draw the line? What about fast computers that are only on 3 hours a day? Giving them "slower" workunits mean that they might not be able to finish the job in time. What about fast computers with a low resource share for SETI@Home? Same problem applies as if they were only powered on 3 hours a day. Also, there is not an even mix of slow and fast to complete workunits in the system at any given moment. Work is split from whatever "tapes" they happen to have available, or whatever was recorded from Arecibo. At what point do you move the line because of new technologies? Who has the time to move that line when it needs to be moved? Preferably it would be a static line that doesn't move, but that's impossible due to the nature of the industry. With so few staff, automation is always a better solution.
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Message 919396 - Posted: 19 Jul 2009, 16:32:34 UTC

Would someone comment on the following idea? There are always bottlenecks in any process, with network bandwidth being everybody's recent focus. But I wonder if seti should approach the problem of un-stable operations by limiting the size of the database. Pick a number, say 3M, and only provide new wu's when the number of results pending drops below that value. Throttling the source of work should make contention-based bottlenecks go away. As is the case now, some people will not get work and will need to use their back-up projects. But the efficiency of the system should go up on a per wu basis. Of course, the productivity will be limited because of the number of wu's issued will be limited. So the threshold number could be adjusted until some sort of optimal value is found, balancing production with inefficient network (or other) operations. I suppose the threshold could be adjusted daily to reflect the wu mix (e.g. shorties/longies/whatever-ies).

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Message 919400 - Posted: 19 Jul 2009, 16:42:38 UTC - in response to Message 919396.

Would someone comment on the following idea? There are always bottlenecks in any process, with network bandwidth being everybody's recent focus. But I wonder if seti should approach the problem of un-stable operations by limiting the size of the database. Pick a number, say 3M, and only provide new wu's when the number of results pending drops below that value. Throttling the source of work should make contention-based bottlenecks go away. As is the case now, some people will not get work and will need to use their back-up projects. But the efficiency of the system should go up on a per wu basis. Of course, the productivity will be limited because of the number of wu's issued will be limited. So the threshold number could be adjusted until some sort of optimal value is found, balancing production with inefficient network (or other) operations. I suppose the threshold could be adjusted daily to reflect the wu mix (e.g. shorties/longies/whatever-ies).

I see where you're going with this, and I know you've been focused on database size for a while. I'm not convinced that is the problem, but the folks in Berkeley would have the metrics.

What worries me is that this won't produce a steady distribution rate, in fact (for multibeam) it would rise and fall by angle range -- we'd see the rate shoot up when we hit a run of shorties, and slow way down with the slowest work units (I've not paid attention to VHAR vs. VLAR, I just know they vary alot).

What I think would be far more beneficial is to smooth the rate work is assigned -- and smooth the rate work is returned.

The problem is that you can't use work assignments to smooth uploads, that has to be a different mechanism.

... but mucking with the feeder would in fact give a pretty smooth rate of assigments, and should be nicely proportional to download bandwidth.
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Message 919427 - Posted: 19 Jul 2009, 17:54:07 UTC - in response to Message 919383.
Last modified: 19 Jul 2009, 17:55:36 UTC

So what about enhancing the pre-filtering of Enhanced WU?
Use two classes for the WU "fast to compute" and "slow to compute"
Tag computers with a "performance class" - again only two classes needed "slow computer" "fast computer".
WU are delivered then delivered on a simple set of rules:
"Fast" computer preferentially gets "slow to compute" WU
"Slow" computer preferentially gets "fast to compute" WU

The net result should be a smoothing out of the time taken for any given WU to be processed, thus giving a smoother demand on the distribution and recovery servers.


The problem is in defining "slow" and "fast" computers. Where do you draw the line? What about fast computers that are only on 3 hours a day? Giving them "slower" workunits mean that they might not be able to finish the job in time. What about fast computers with a low resource share for SETI@Home? Same problem applies as if they were only powered on 3 hours a day. Also, there is not an even mix of slow and fast to complete workunits in the system at any given moment. Work is split from whatever "tapes" they happen to have available, or whatever was recorded from Arecibo. At what point do you move the line because of new technologies? Who has the time to move that line when it needs to be moved? Preferably it would be a static line that doesn't move, but that's impossible due to the nature of the industry. With so few staff, automation is always a better solution.


I was hoping someone else might define "fast" and "slow" computers, but to no avail:-(
So for a kick around - fast greater than 10 random WU returned per day.
and slow - less than, or exactly 10 random WU returned per day.
On a per-processor basis.

As I say, a starter......

As to rate of update of the rule - not very often, I would expect a six month life at the very least, once it was initially tuned
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Message 919433 - Posted: 19 Jul 2009, 18:03:02 UTC - in response to Message 919374.
Last modified: 19 Jul 2009, 18:03:26 UTC


So what about enhancing the pre-filtering of Enhanced WU?
Use two classes for the WU "fast to compute" and "slow to compute"
Tag computers with a "performance class" - again only two classes needed "slow computer" "fast computer".
WU are delivered then delivered on a simple set of rules:
"Fast" computer preferentially gets "slow to compute" WU
"Slow" computer preferentially gets "fast to compute" WU

The net result should be a smoothing out of the time taken for any given WU to be processed, thus giving a smoother demand on the distribution and recovery servers.

I've read this about four times, and I don't think it would do what you expect.

What it does is take some of the pending uploads from the fastest computers and moves them to the slowest computers.

The argument being that the fast work will stay out longer because it is on the slow computers.

The problem is that there are a lot of slow computers, and while they might be ten times slower than the mega-crunchers, I suspect they make up for that in sheer numbers (ten times as many of them), and we're back in the same spot.

That depends on the definition of "fast" and "slow" of course. Draw the line too high, and you have too many slow machines and the slow work comes back even faster, draw it too low and the fast computers still dominate.

It is much easier to move a problem around than it is to solve it.
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Message 919452 - Posted: 19 Jul 2009, 19:29:38 UTC - in response to Message 919433.

The problem is that for days on end the system is in constant re-try, during which time the pile of results waiting to be returned grows. This has two effects, the servers spend more of the available band width on sending out "reject request" messages of various forms, and because few WU are being returned few are being requested, and a backlog of new WU is developing.

By moderating the return rate (WU reporting) by delivering "computation time constant" (real hours to complete) data the reject rate will drop, thus the "wasted" band width will be reduced, and the smoother the flow through the servers will be in both directions. For maximised throughput you are really looking for constant flow rates, surging flow rates cause all sorts of turbulence.

The definitions of "fast" and "slow" computers would obviously need to be carefully considered, and in this case it will not be the absolute clock speed that is the determining factor, but the long term processing ability - how many WU per day are processed being the more realistic indicator of "speed" in this sort of situation.

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Message 919460 - Posted: 19 Jul 2009, 19:43:29 UTC - in response to Message 919452.

The problem is that for days on end the system is in constant re-try, during which time the pile of results waiting to be returned grows.

That definitely makes sense to me. We can make uploads more efficient by keeping the clients from hammering the upload server.

The reason I don't think controlling work assignments will help with uploads is latency.

When a work unit goes out, it's going to come back anywhere between a few minutes (fastest CUDA) to a few weeks (longest AP, big cache, CPU, vacation, etc.), and once it is released, that's the last the project knows.

Slowing down work today will help days from now, but to help with today we needed to throttle last week.

One way to help moderate the uploads (this has already been described elsewhere) is to treat one failed upload like all of the pending uploads failed. That keeps the system with a bunch of work units from retrying each one when failure is inevitable. To make sure the "best" work unit uploads first, retry in deadline order, shortest to longest.

If you have fifty pending uploads, this reduces the server load from your cruncher by a factor of fifty.

This is already in the works, I understand.

The next stage is to be able to tell the BOINC clients to slow down, to moderate the upload rate.

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Message 919462 - Posted: 19 Jul 2009, 19:49:15 UTC - in response to Message 919213.

The easiest way to do this would be to cap download bandwidth at the router. Is there traffic shaping imposed? If not, I would be shocked, as this is the quickest and easiest way to help the situation (assuming the router(s) in place have this capability).

Bandwidth shaping is probably the worst way.

Why? When you induce latency, connections at the server last longer, and finish more slowly.

It is widely done because frequently, you can't go back to the server(s) or users and adjust them, shaping at the source isn't available.

Shaping is the best option when it is the only option.

What you want is exactly enough active downloads to completely fill the pipe, and for those downloads to run at the highest possible speed -- and then get out of the way for the next upload.

Fewer simultaneous downloads, fewer connections, fewer open files, lower server load.

... but same number of bits/second, and same number of connections per hour.

Fiddling with the feeder lets the splitter run at top speed, and basically restricts bandwidth at the scheduler.

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Message 919463 - Posted: 19 Jul 2009, 19:49:28 UTC - in response to Message 919460.

I've just noticed that the Upload server "Bruno" is disabled, which may be part of today's little problem :-((
No or reduced upload server capacity = upload log jam in what ever language.
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Message 919472 - Posted: 19 Jul 2009, 20:17:39 UTC - in response to Message 919463.

I've just noticed that the Upload server "Bruno" is disabled, which may be part of today's little problem :-((
No or reduced upload server capacity = upload log jam in what ever language.


the upload server is disabled all day, and was (at my oppinion) already instable yesterday (it took a couple of tries to upload my stuff yesterday)

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Message 919502 - Posted: 19 Jul 2009, 22:04:02 UTC


Matt, you are come back tomorrow from your vacation for to 'kick' the server? :-)

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Message 919532 - Posted: 19 Jul 2009, 23:16:56 UTC

Without going back through pages of posts, it was once mentioned that the data highway from the main point of campus entry to SETI servers is the bottleneck. From more recent posts it appears installing a dedicated, multi-fiber cable and high-speed routers, thereby shoving the headache out to the street or the antenna, wouldn’t cure the bottleneck. Then there is the sense from some posts that campus Admin (1) places a low priority on the project, that there is a (2) factor contaminating incoming signal with radar pulses, (3) software is being rewritten by a single volunteer who is currently on a well-deserved vacation, and (4) correct me if I'm wrong, simply throwing more servers into the mix won't cure it either. Then, there is also the undertone that crunchers are somehow to blame and they need to go somewhere else to volunteer their multicores.

That said, what's the chance of obtaining a comprehensive wish list outlining the specifics, an estimate of the damage to implement those upgrades, and some indication that if supplied, Admin won’t appropriate the money for something they consider more academically appropriate? Since crunchers are only going to expand their abilities to process, this seems a better choice than evolving toward an egalitarian, membership-only distribution based on single 286 core equivalencies. Perhaps a promising student from the business school might inspect this as a graduate analysis paper and evaluate the potential revenue stream before Google - bless their entrepreneurial talents - builds a 100 mile square of signal gathering ears, dedicates an inexhaustible supply of software engineers, and utilizes surplus crunching from their subscriber net for business analytics in lieu of user fees. If this is being taken as criticism, the whole point has been missed.

The point is that the whole world participates in this project, has great interest in its success, simply wants empirical evidence that we're not alone, and, I believe, would pony up for costs directed toward meeting identified goals (like helping with volunteer tuition for a start). To see this project devolve before a world audience isn't an acceptable outcome and, as unfair as it might seem, failure to embrace challenges doesn't bode well for Berkeley.
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Message 919568 - Posted: 20 Jul 2009, 1:09:49 UTC - in response to Message 919502.

Matt, you are come back tomorrow from your vacation for to 'kick' the server? :-)

No, Matt is not coming back tomorrow, he will be on vacation next week.
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Message 919584 - Posted: 20 Jul 2009, 1:35:41 UTC - in response to Message 919568.

The upload server is back running.
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