HFSC explained - decoupled bandwidth and delay - Q&A - Ask anything
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ok i understood realtime now, basically it can be used in cases where u know the type of traffic and its bitrate so then can reduce the latency on it.
one thing i didnt understand regarding the other link u gave me is do we set linkshare on the root queue or just the bandwdith
below is my current setup, can u tell me if its fine or simply point out whats wrong
WAN - bandwidth 100Mb
-qInternet - bandwidth 3000Kb - UL 3000Kb - LS 3000Kb - codel
–qACK - bandwidth 30% - LS 30% - codel - priority 6
--qOthersDefault - bandwidth 10% - LS 10% - codel - priority 4
--qP2P - bandwidth 5% - LS 5% - codel - priority 1 - default
--qVoIP - bandwidth 45% - RT 447Kb/5/224Kb - LS 45% - codel - priority 7
--qOthersHigh - bandwidth 10% - LS 10% - codel - priority 5LAN - bandwidth 100Mb
-qInternet - bandwidth 12000Kb - UL 12000Kb - LS 12000Kb - codel
--qACK - bandwidth 15% - LS 15% - codel - priority 6
--qOthersDefault - bandwidth 50% - LS 50% - codel - priority 4
--qP2P - bandwidth 5% - LS 5% - codel - priority 1 - default
--qVoIP - bandwidth 20% - RT 447Kb/5/224Kb - LS 20% - codel - priority 7
--qOthersHigh - bandwidth 10% - LS 10% - codel - priority 5 -
Well I don't understand what's wrong in using it, all it does is give that queue a minium value all the time compared to link share which would give that queue the set bandwidth when it's back logged, and in the case of voip you need to have real-time bandwidth available as soon as the call starts so the signaling isn't delayed which would inturn increase call setup and RTP time. Even if there is no traffic for that queue which has real-time set that bandwidth isn't lost and still available for other queues.
In voip we at least know the packet size and what interval it's sent so I guess it's straight forward to set a proper value for real-time
- Realtime always comes from the root queue, and can be greater than the parent queue's available bandwidth and even upperlimit, allowing you to starve your parent queue
- Realtime counts bandwidth consumed above your real-time negatively against you and will lower your bandwidth, violating the "minimum" exception. Rule of thumb, NEVER allow a queue with realtime to exceed it's allotted realtime bandwidth
- Link-share already gives me "immediate" bandwidth. I currently cannot measure a difference between my link being saturated or idle to within an accuracy or 0.01ms and 0.001% loss, and I only use linkshare.
Likshare has 99% of the benefit and none of the dangers. The primary benefit of realtime seems that it makes it simpler to not have to think about managing parent queues, except that simplicity comes at the cost of possibly harming the parent queues.
They added realtime for a reason, but without knowing more about the reason and implementation, I cannot empirically measure a benefit over linkshare. There may be some benefit for very slow links where the MTU is relatively large compared to the bandwidth
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Well I don't quiet agree with you on the theory that real-time would take from root instead of the parent queue because if that were the case then parent queue would hold no importance and all the tests I did so far connecting to a voip server which is almost 250ms away in a different continent it doesn't seem real-time borrows from root queue and starves parent queue when I saturated the line. The limit on the parent queue comes in effect and the root queue doesn't exceed the traffic set as upper limit on the parent of the queue which I set real-time on. In fact after setting real-time on my voip queue I find the calls connect more quickly, jitter reduced and with active torrents saturating the line I didn't see any quality drops in voip but in fact voip experience improved.
The main thing is to know the proper bitrate and set proper values in real-time if you use that on any queue and so far what i read no where does it say real-time would borrow from root queue going over the limit that the parent queue has set.
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Thanks for bumping this thread up. (it should be a sticky next to ermal's post which is something like 7 years old now)
Anyway, is it a limitation of the GUI to have only one interface per HFSC queue? This would solve the issue of splitting bandwidth using UPPERLIMIT on multiple LAN scenarios? Really knocks the flexibility of link share around when bandwidth is carved up on a per interface level.
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Well I don't quiet agree with you on the theory that real-time would take from root instead of the parent queue..
It's not debatable, it's a fact.
queue root_igb0 on igb0 bandwidth 99Mb priority 0 {qACK, qUnclassified, qClassified}
queue qACK on igb0 bandwidth 19.80Mb qlimit 1024
queue qUnclassified on igb0 bandwidth 29.70Mb {qUDP, qDefault}
queue qUDP on igb0 bandwidth 13.07Mb qlimit 1024 hfsc( codel linkshare(16.34Mb 5 13.07Mb) )
queue qDefault on igb0 bandwidth 13.07Mb qlimit 1024 hfsc( codel default )
queue qClassified on igb0 bandwidth 1Mb {qNormal, qHigh}
queue qNormal on igb0 bandwidth 440Kb qlimit 1024 hfsc( codel )
queue qHigh on igb0 bandwidth 440Kb qlimit 1024 hfsc( codel realtime 40Mb linkshare(550Kb 5 440Kb) )Notice the parent queue qClassified is assigned 1Mb of bandwidth and the child queue qHigh is assigned 440Kb of linkshare and 40Mb of realtime. This is a perfectly valid configuration. And if qHigh decided to pull down 40Mb a sec and qClassified only has 1Mb of bandwidth, how much bandwidth is left over for qNormal? None.
Realtime is ALWAYS guaranteed to be available, assuming the queue has not gone over the provisioned amount of realtime, and all bandwidth the realtime consumes counts towards the parent queue(s).
And of course your realtime didn't starve the parent queue in your simple test because realtime cannot consume more than 80% of the bandwidth. That means there's always at least 20% free for link share. As long as there is some free bandwidth and you don't have an upper limit on the parent queue that is being exceeded by the realtime child queue, the sibling queues will still have some bandwidth to work with.
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Whoops I feel like my question above may have also been irrelevant to the specific topic of this thread. (discussing decoupled bw/delay)
Perhaps a more direct question about the thread topic would be on the ping test by Nullity (thanks for that hands on example)…
How would CODEL affect that test, if the goal for CODEL is to maintain a 5ms buffer length, the packets in that test are essentially held for 10ms (the d parameter) due to the 0Kb bandwidth of the M1 parameter?
Would CODEL just drop all the packets? And if not, why? (Unfortunately I don't have access to a pfsense lab environment where I can run interesting tests like these.)
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Whoops I feel like my question above may have also been irrelevant to the specific topic of this thread. (discussing decoupled bw/delay)
Perhaps a more direct question about the thread topic would be on the ping test by Nullity (thanks for that hands on example)…
How would CODEL affect that test, if the goal for CODEL is to maintain a 5ms buffer length, the packets in that test are essentially held for 10ms (the d parameter) due to the 0Kb bandwidth of the M1 parameter?
Would CODEL just drop all the packets? And if not, why? (Unfortunately I don't have access to a pfsense lab environment where I can run interesting tests like these.)
CoDel is designed to improve delay but keep all other aspects unchanged (like bandwidth). CoDel keeps the queue size low, but at least 1 packet needs to be queued before CoDel drops other incoming packets.
Artificially adding delay with HFSC (or ipfw once the CoDel & fq_codel are added: http://caia.swin.edu.au/freebsd/aqm/) might affect CoDel's queue delay calculation, or maybe not since I don't know the implementation details. Regardless, CoDel should not cause a problem, by design, but the easiest answer is to just run a test a see. :)
I really need to set up a (virtual?) lab too…
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Codel has an extremely large internal buffer, but the buffering algorithm is does not do a hard cut-off. When the latency gets greater than 10ms, it will drop one packet, not all packets. It will then check one interval later to see if the buffer has gotten below the threshold. If not, drop another packet.
When playing around, I was able to get Codel to cause multi-second ping times, but you really have to game it.
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Advanced real-time VOIP configuration
Say you want to efficiently improve your VOIP setup. You have 7 VOIP lines and a 5Mbit upload. Each VOIP packet is 160 bytes (G.711), with an average bitrate of 64Kbit/s per line (a 160byte packet sent every 20ms). We want to improve our worst-case delay (which also improves worst-case jitter and overall call quality) from 20ms to 5ms, so we calculate the bitrate needed to achieve that:
160 bytes × 8 = 1280 bits
1280 bits × (1000ms ÷ 5ms) = 256000 bits/sSo, to send a 160 byte packet within 5ms we need to allocate 256Kbit/s. This gives us our m1 (256Kb) and our d (5).
Now we need to calculate our maximum average bitrate:
7 lines × 64Kbit/s = 448Kbit/s
Just to be safe, we allocate bandwidth for an extra line, so
8 lines × 64Kbit/s = 512Kbit/sThis gives us our m2 (512Kb).
To make sure that your m1 (the per packet delay) can always be fulfilled by your connection, make sure that the m1 (256Kb) multiplied by the maximum number of simultaneous sessions (7 or 8) is less than your maximum upload. 2048Kbit (256Kb × 8) is less than 5000Kbit, good.
Our finalized configuration is:
m1=256Kb
d=5
m2=512KbThis configuration will guarantee a 7.4ms (5ms+MTU transmission delay) worst-case delay for each packet, with a limit of 8 simultaneous calls (512Kbit/s). We get low-delay, low-jitter calls as though you had allocated 2048Kbit/s of bandwidth, but you actually only allocated 512Kbit/s of bandwidth.
I may be misunderstanding the m1 value. It would seem to me that with m1 set to 256Kb with packets taking ~7.5ms to send if I had 8 active lines and by chance each 20ms packet happened at the same time, I would have a potential 80ms (8 x 7.5) delay as the queue becomes backlogged. While not disastrous to VoIP, if this scales up I would begin to see the delays in the call. Perhaps my understanding of VoIP is lacking.
Sorry about the zombie thread…
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I think it's simpler to just use the single packet case. If you have 1 160byte(1280bit) packet every 50ms, that's 64kbit/s. But, at 64Kb/s, HFSC is allowed to schedule that packet in any manner, as long as it completes before the 50ms to send the next packet. This means your absolutely worst case is 50ms.
If you want this packet to be sent out faster, say 5ms, then you need "5ms" of bandwidth for scheduling reasons. That will take 256Kb of bandwidth to send 1280bit packet in 5ms. But you don't need 256Kb average, just 256Kb of burst. So you tell HFSC, 64Kb average, 256Kb burst for 5ms.
Or something along these lines.
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Advanced real-time VOIP configuration
Say you want to efficiently improve your VOIP setup. You have 7 VOIP lines and a 5Mbit upload. Each VOIP packet is 160 bytes (G.711), with an average bitrate of 64Kbit/s per line (a 160byte packet sent every 20ms). We want to improve our worst-case delay (which also improves worst-case jitter and overall call quality) from 20ms to 5ms, so we calculate the bitrate needed to achieve that:
160 bytes × 8 = 1280 bits
1280 bits × (1000ms ÷ 5ms) = 256000 bits/sSo, to send a 160 byte packet within 5ms we need to allocate 256Kbit/s. This gives us our m1 (256Kb) and our d (5).
Now we need to calculate our maximum average bitrate:
7 lines × 64Kbit/s = 448Kbit/s
Just to be safe, we allocate bandwidth for an extra line, so
8 lines × 64Kbit/s = 512Kbit/sThis gives us our m2 (512Kb).
To make sure that your m1 (the per packet delay) can always be fulfilled by your connection, make sure that the m1 (256Kb) multiplied by the maximum number of simultaneous sessions (7 or 8) is less than your maximum upload. 2048Kbit (256Kb × 8) is less than 5000Kbit, good.
Our finalized configuration is:
m1=256Kb
d=5
m2=512KbThis configuration will guarantee a 7.4ms (5ms+MTU transmission delay) worst-case delay for each packet, with a limit of 8 simultaneous calls (512Kbit/s). We get low-delay, low-jitter calls as though you had allocated 2048Kbit/s of bandwidth, but you actually only allocated 512Kbit/s of bandwidth.
I may be misunderstanding the m1 value. It would seem to me that with m1 set to 256Kb with packets taking ~7.5ms to send if I had 8 active lines and by chance each 20ms packet happened at the same time, I would have a potential 80ms (8 x 7.5) delay as the queue becomes backlogged. While not disastrous to VoIP, if this scales up I would begin to see the delays in the call. Perhaps my understanding of VoIP is lacking.
Sorry about the zombie thread…
The scenerio was for 7 lines, not 8. It could handle 8, but that'd be 100% usage… I dunno what standard operating prodecure is, but allowing for no overhead seems dangerous.
There will be no problematic backlog. The scenerio above can handle 7 simultaneous lines while guaranteeing a per-packet latency of ~7.5ms for every VOIP packet, because both the internet connection and queue are not overused at any point, even during 7 simultaneous VOIP calls.
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The scenerio was for 7 lines, not 8. It could handle 8, but that'd be 100% usage… I dunno what standard operating prodecure is, but allowing for no overhead seems dangerous.
There will be no problematic backlog. The scenerio above can handle 7 simultaneous lines while guaranteeing a per-packet latency of ~7.5ms for every VOIP packet, because both the internet connection and queue are not overused at any point, even during 7 simultaneous VOIP calls.
Thanks for the response! It sounds like the m1 is per packet. Is that correct? So if pfSense received 2 packets within 2ms each packet would be allocated m1 and I would see a burst of 256Kb/s x 2 = 512Kb/s?
You also talk about the connection not being over used. If I have a 5Mb/s upload (rated for 6Mb/s), 4Mb/s for linkshare in other queues (4Mb/s upper limit), and 1Mb/s for the qVoIP, I should never have to worry about over saturation.
Note: Not exact numbers but the same idea. -
Thanks for the response! It sounds like the m1 is per packet. Is that correct? So if pfSense received 2 packets within 2ms each packet would be allocated m1 and I would see a burst of 256Kb/s x 2 = 512Kb/s?
Yeah, exactly. As I understand it m1/d are per packet. If m1/d were per queue, then the per packet latency would be dependant on how many packets were queued, which would make latency unpredictable and practically break HFSC's latency guarantees.
You also talk about the connection not being over used. If I have a 5Mb/s upload (rated for 6Mb/s), 4Mb/s for linkshare in other queues (4Mb/s upper limit), and 1Mb/s for the qVoIP, I should never have to worry about over saturation.
Note: Not exact numbers but the same idea.There are a number of ways to do it, but that setup seems fine. You should stress test it to confirm proper functionality though, regardless.
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It's kind of both. The scheduling is done at the queue level, but it is done in discrete steps of packets. The service curve is just the priority, which every curve is "greater" at any given moment is scheduled next. The m1+d modify the curve to make it look like it is greater. If sending the packet will push it beyond its limit, it won't send the packet yet. Of course if there is spare bandwidth and no other packets, it'll just send the packet anyway.
I just think of HFSC as a dynamic priority based queue where the priority is adjusted in real-time per packet processed, such that all queue configurations are respected. Of course my simplification breaks down in extreme situations, like absurdly low bandwidths, absurdly large MTUs, or absurdly low target latencies, but it's a good rule of thumb.
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It's kind of both. The scheduling is done at the queue level, but it is done in discrete steps of packets. The service curve is just the priority, which every curve is "greater" at any given moment is scheduled next. The m1+d modify the curve to make it look like it is greater. If sending the packet will push it beyond its limit, it won't send the packet yet. Of course if there is spare bandwidth and no other packets, it'll just send the packet anyway.
I just think of HFSC as a dynamic priority based queue where the priority is adjusted in real-time per packet processed, such that all queue configurations are respected. Of course my simplification breaks down in extreme situations, like absurdly low bandwidths, absurdly large MTUs, or absurdly low target latencies, but it's a good rule of thumb.
(Using the scenerio from earlier.)
If 8 packets are instantly queued and m1/d was per queue, the queue would only get 256Kbit for 5ms which would only be 1 packet transmitted, violating the HFSC's latency guarantees for the other 7 packets.If it were per queue we'd need to set m1 to 8x the bandwidth to support 8 concurrent sessions, which may be true, but I think m1/d is per packet since HFSC's m1/d/m2 correspond with the umax/dmax/r, which is defined in page 10 the HFSC paper as follows:
Each session is characterized by three parameters: the largest unit of work, denoted umax, for which the session requires delay guarantee, the guaranteed delay dmax, and the session's average rate r. As an example, if a session requires per packet delay guarantee, then umax represents the maximum size of a packet. Similarly, a video or an audio session can require per frame delay guarantee, by setting umax to the maximum size of a frame.
umax = packet and umax/dmax directly correspond with m1/d, therefore m1/d are per packet as well.
Edit: Also, I'm not sure what "session" means exactly… maybe flow?
I don't understand the HFSC paper very well, but the few parts I do understand only mention single session (flow?) queues, which may be the only way to to properly use HFSC, but that seems strangely limited.We'd had this per packet/queue argument before and I had few more references, but I forget them now. Maybe they're in this thread? Reading the source-code or even the pseudo-code found in the HFSC paper could answer our question, but it seems too advanced for me...
Testing this per queue/per packet theory should be reasonably easy. Someday... :-\
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(Using the scenerio from earlier.)
If 8 packets are instantly queued and m1/d was per queue, the queue would only get 256Kbit for 5ms which would only be 1 packet transmitted, violating the HFSC's latency guarantees for the other 7 packets.Latency guarantees are only guaranteed if you don't go past your bandwidth limit. If you burst 8 1280bit(160byte) packets at once, the last packet is going to have 8960bits of data in front of it and 10,240 including itself. To send 10240 bits in 5ms, you need 2mbits of bandwidth. You can't have your cake and eat it to. It is impossible in this universe to guarantee all 8 packets will get sent within 5ms with anything less than ~2Mb/s.
Even in this case, the average bandwidth is 1280bits * 20/sec(every 50ms) * 8 packets = 204,800bps, which is about 1/10th the bandwidth you need to make sure none of those 8 packets hang in the queue for more than 5ms. You need about 2Mb/s, but you only need it for 5ms. Your average is 204.8Kb/s.
m1 = burst
d = duration of burst (of course you can't send fraction packets, so you need to make sure all the packets you want to burst fit m1*d)
m2 = overall averagem1 is moot if you attempt to go over m2.
*my interpreation
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(Using the scenerio from earlier.)
If 8 packets are instantly queued and m1/d was per queue, the queue would only get 256Kbit for 5ms which would only be 1 packet transmitted, violating the HFSC's latency guarantees for the other 7 packets.Latency guarantees are only guaranteed if you don't go past your bandwidth limit. If you burst 8 1280bit(160byte) packets at once, the last packet is going to have 8960bits of data in front of it and 10,240 including itself. To send 10240 bits in 5ms, you need 2mbits of bandwidth. You can't have your cake and eat it to. It is impossible in this universe to guarantee all 8 packets will get sent within 5ms with anything less than ~2Mb/s.
Even in this case, the average bandwidth is 1280bits * 20/sec(every 50ms) * 8 packets = 204,800bps, which is about 1/10th the bandwidth you need to make sure none of those 8 packets hang in the queue for more than 5ms. You need about 2Mb/s, but you only need it for 5ms. Your average is 204.8Kb/s.
m1 = burst
d = duration of burst (of course you can't send fraction packets, so you need to make sure all the packets you want to burst fit m1*d)
m2 = overall averagem1 is moot if you attempt to go over m2.
*my interpreation
Any response to the HFSC paper saying that m1/d & umax/dmax are per packet?
It's confusing since m2 is obviously per queue.
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(Using the scenerio from earlier.)
If 8 packets are instantly queued and m1/d was per queue, the queue would only get 256Kbit for 5ms which would only be 1 packet transmitted, violating the HFSC's latency guarantees for the other 7 packets.Latency guarantees are only guaranteed if you don't go past your bandwidth limit. If you burst 8 1280bit(160byte) packets at once, the last packet is going to have 8960bits of data in front of it and 10,240 including itself. To send 10240 bits in 5ms, you need 2mbits of bandwidth. You can't have your cake and eat it to. It is impossible in this universe to guarantee all 8 packets will get sent within 5ms with anything less than ~2Mb/s.
Even in this case, the average bandwidth is 1280bits * 20/sec(every 50ms) * 8 packets = 204,800bps, which is about 1/10th the bandwidth you need to make sure none of those 8 packets hang in the queue for more than 5ms. You need about 2Mb/s, but you only need it for 5ms. Your average is 204.8Kb/s.
m1 = burst
d = duration of burst (of course you can't send fraction packets, so you need to make sure all the packets you want to burst fit m1*d)
m2 = overall averagem1 is moot if you attempt to go over m2.
*my interpreation
Any response to the HFSC paper saying that m1/d & umax/dmax are per packet?
It's confusing since m2 is obviously per queue.
Well, actually, it doesn't exactly say it's per-packet. It just says "if a session requires per packet delay guarantee, then umax represents the maximum size of a packet"… so, would 7 separate VOIP flows be considered a single session or multiple sessions? Is a queue's traffic considered a single "session", regardless?
Fuck, I really don't want to read the HFSC paper for the 100th time... Though, IIRC, the paper only mentions single-flow scenerios, so the source-code might provide the only proper answer.
I'm beginning to think you may be right though; m1 should be multiplied × 7 to support 7 concurrent flows.
HFSC... :-\
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Again From: http://linux-tc-notes.sourceforge.net/tc/doc/sch_hfsc.txt
Example 3.
That worked, but not as well as you hoped. You recall TCP can
send multiple packets before waiting for a reply and you would
like them all sent before web gets to send it's first packet.
You also estimate you have 10 concurrent users all doing the same
thing. From a packet capture you decide sending 4 packets before
waiting for a reply is typical.10 users by 4 packets each means 40 MTU sized packets. Thus you
must adjust the burst speed, so ssh gets 40 times the speed of
web and you must allow for 40 MTU sized packets in the burst
time:tc class add dev eth0 parent 1:0 classid 1:1 hfsc
ls m1 24kbps d 60ms m2 500kbps # web
tc class add dev eth0 parent 1:0 classid 1:2 hfsc
ls m1 975kbps d 60ms m2 500kbps # sshNote that 975:24 is 40:1, and (40 * 1500 / 1mbps = 60).
Seems to imply the same. Though I think that math might be wrong. Seems like it should be (40 * 1500 * 8 / 1mbps = 457ms) (A 1500B MTU is 12000 not 1500)
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Again From: http://linux-tc-notes.sourceforge.net/tc/doc/sch_hfsc.txt
Example 3.
That worked, but not as well as you hoped. You recall TCP can
send multiple packets before waiting for a reply and you would
like them all sent before web gets to send it's first packet.
You also estimate you have 10 concurrent users all doing the same
thing. From a packet capture you decide sending 4 packets before
waiting for a reply is typical.10 users by 4 packets each means 40 MTU sized packets. Thus you
must adjust the burst speed, so ssh gets 40 times the speed of
web and you must allow for 40 MTU sized packets in the burst
time:tc class add dev eth0 parent 1:0 classid 1:1 hfsc
ls m1 24kbps d 60ms m2 500kbps # web
tc class add dev eth0 parent 1:0 classid 1:2 hfsc
ls m1 975kbps d 60ms m2 500kbps # sshNote that 975:24 is 40:1, and (40 * 1500 / 1mbps = 60).
Seems to imply the same. Though I think that math might be wrong. Seems like it should be (40 * 1500 * 8 / 1mbps = 457ms) (A 1500B MTU is 12000 not 1500)
Yeah, he makes that mistake in example 2 as well. I tried emailing him months ago but the email bounced.
I guess it makes more sense if m1/m2 were both per queue.