[Codel] [Bloat] [Make-wifi-fast] TCP_NOTSENT_LOWAT applied to e2e TCP msg latency
Eric Dumazet
eric.dumazet at gmail.com
Wed Oct 27 01:40:26 EDT 2021
On 10/26/21 8:45 PM, Bob McMahon wrote:
> This is linux. The code flow is burst writes until the burst size, take a timestamp, call select(), take second timestamp and insert time delta into histogram, await clock_nanosleep() to schedule the next burst. (actually, the deltas, inserts into the histogram and user i/o are done in another thread, i.e. iperf 2's reporter thread.)
>
> I still must be missing something. Does anything else need to be set to reduce the skb size? Everything seems to be indicating 4K writes even when gso_max_size is 2000 (I assume these are units of bytes?) There are ten writes, ten reads and ten RTTs for the bursts. I don't see partial writes at the app level.
>
> [root at localhost iperf2-code]# ip link set dev eth1 gso_max_size 2000
You could check with tcpdump on eth1, that outgoing packets are no longer 'TSO/GSO', but single MSS ones.
(Note: this device gso_max_size is only taken into account for flows established after the change)
>
> [root at localhost iperf2-code]# ip -d link sh dev eth1
> 9: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UNKNOWN mode DEFAULT group default qlen 1000
> link/ether 00:90:4c:40:04:59 brd ff:ff:ff:ff:ff:ff promiscuity 0 minmtu 68 maxmtu 1500 addrgenmode eui64 numtxqueues 1 numrxqueues 1 gso_max_size 2000 gso_max_segs 65535
> [root at localhost iperf2-code]# uname -r
> 5.0.9-301.fc30.x86_64
>
>
> It looks like RTT is being driven by WiFi TXOPs as doubling the write size increases the aggregation by two but has no significant effect on the RTTs.
>
> 4K writes: tot_mpdus 328 tot_ampdus 209 mpduperampdu 2
>
>
> 8k writes: tot_mpdus 317 tot_ampdus 107 mpduperampdu 3
>
>
> [root at localhost iperf2-code]# src/iperf -c 192.168.1.1%eth1 --trip-times -i 1 -e --tcp-write-prefetch 4 -l 4K --burst-size=40K --histograms
> WARN: option of --burst-size without --burst-period defaults --burst-period to 1 second
> ------------------------------------------------------------
> Client connecting to 192.168.1.1, TCP port 5001 with pid 5145 via eth1 (1 flows)
> Write buffer size: 4096 Byte
> Bursting: 40.0 KByte every 1.00 seconds
> TCP window size: 85.0 KByte (default)
> Event based writes (pending queue watermark at 4 bytes)
> Enabled select histograms bin-width=0.100 ms, bins=10000
> ------------------------------------------------------------
> [ 1] local 192.168.1.4%eth1 port 45680 connected with 192.168.1.1 port 5001 (MSS=1448) (prefetch=4) (trip-times) (sock=3) (ct=5.30 ms) on 2021-10-26 20:25:29 (PDT)
> [ ID] Interval Transfer Bandwidth Write/Err Rtry Cwnd/RTT NetPwr
> [ 1] 0.00-1.00 sec 40.1 KBytes 329 Kbits/sec 11/0 0 14K/10091 us 4
> [ 1] 0.00-1.00 sec S8-PDF: bin(w=100us):cnt(10)=1:1,36:1,40:1,44:1,46:1,48:1,49:1,50:2,52:1 (5.00/95.00/99.7%=1/52/52,Outliers=0,obl/obu=0/0) (5.121 ms/1635305129.152339)
> [ 1] 1.00-2.00 sec 40.0 KBytes 328 Kbits/sec 10/0 0 14K/4990 us 8
> [ 1] 1.00-2.00 sec S8-PDF: bin(w=100us):cnt(10)=1:2,39:1,45:1,49:5,50:1 (5.00/95.00/99.7%=1/50/50,Outliers=0,obl/obu=0/0) (4.991 ms/1635305130.153330)
> [ 1] 2.00-3.00 sec 40.0 KBytes 328 Kbits/sec 10/0 0 14K/4904 us 8
> [ 1] 2.00-3.00 sec S8-PDF: bin(w=100us):cnt(10)=1:2,29:1,49:4,50:1,59:1,75:1 (5.00/95.00/99.7%=1/75/75,Outliers=0,obl/obu=0/0) (7.455 ms/1635305131.147353)
> [ 1] 3.00-4.00 sec 40.0 KBytes 328 Kbits/sec 10/0 0 14K/4964 us 8
> [ 1] 3.00-4.00 sec S8-PDF: bin(w=100us):cnt(10)=1:2,49:4,50:2,59:1,65:1 (5.00/95.00/99.7%=1/65/65,Outliers=0,obl/obu=0/0) (6.460 ms/1635305132.146338)
> [ 1] 4.00-5.00 sec 40.0 KBytes 328 Kbits/sec 10/0 0 14K/4970 us 8
> [ 1] 4.00-5.00 sec S8-PDF: bin(w=100us):cnt(10)=1:2,49:6,59:1,65:1 (5.00/95.00/99.7%=1/65/65,Outliers=0,obl/obu=0/0) (6.404 ms/1635305133.146335)
> [ 1] 5.00-6.00 sec 40.0 KBytes 328 Kbits/sec 10/0 0 14K/4986 us 8
> [ 1] 5.00-6.00 sec S8-PDF: bin(w=100us):cnt(10)=1:2,48:1,49:1,50:4,59:1,64:1 (5.00/95.00/99.7%=1/64/64,Outliers=0,obl/obu=0/0) (6.395 ms/1635305134.146343)
> [ 1] 6.00-7.00 sec 40.0 KBytes 328 Kbits/sec 10/0 0 14K/5059 us 8
> [ 1] 6.00-7.00 sec S8-PDF: bin(w=100us):cnt(10)=1:2,39:1,49:3,50:2,60:1,85:1 (5.00/95.00/99.7%=1/85/85,Outliers=0,obl/obu=0/0) (8.417 ms/1635305135.148343)
> [ 1] 7.00-8.00 sec 40.0 KBytes 328 Kbits/sec 10/0 0 14K/5407 us 8
> [ 1] 7.00-8.00 sec S8-PDF: bin(w=100us):cnt(10)=1:2,40:1,49:4,50:1,59:1,75:1 (5.00/95.00/99.7%=1/75/75,Outliers=0,obl/obu=0/0) (7.428 ms/1635305136.147343)
> [ 1] 8.00-9.00 sec 40.0 KBytes 328 Kbits/sec 10/0 0 14K/5188 us 8
> [ 1] 8.00-9.00 sec S8-PDF: bin(w=100us):cnt(10)=1:2,40:1,49:3,50:3,64:1 (5.00/95.00/99.7%=1/64/64,Outliers=0,obl/obu=0/0) (6.388 ms/1635305137.146284)
> [ 1] 9.00-10.00 sec 40.0 KBytes 328 Kbits/sec 10/0 0 14K/5306 us 8
> [ 1] 9.00-10.00 sec S8-PDF: bin(w=100us):cnt(10)=1:2,39:1,49:2,50:2,51:1,60:1,65:1 (5.00/95.00/99.7%=1/65/65,Outliers=0,obl/obu=0/0) (6.422 ms/1635305138.146316)
> [ 1] 0.00-10.01 sec 400 KBytes 327 Kbits/sec 102/0 0 14K/5939 us 7
> [ 1] 0.00-10.01 sec S8(f)-PDF: bin(w=100us):cnt(100)=1:19,29:1,36:1,39:3,40:3,44:1,45:1,46:1,48:2,49:33,50:18,51:1,52:1,59:5,60:2,64:2,65:3,75:2,85:1 (5.00/95.00/99.7%=1/65/85,Outliers=0,obl/obu=0/0) (8.417 ms/1635305135.148343)
>
> [root at localhost iperf2-code]# src/iperf -s -i 1 -e -B 192.168.1.1%eth1
> ------------------------------------------------------------
> Server listening on TCP port 5001 with pid 6287
> Binding to local address 192.168.1.1 and iface eth1
> Read buffer size: 128 KByte (Dist bin width=16.0 KByte)
> TCP window size: 128 KByte (default)
> ------------------------------------------------------------
> [ 1] local 192.168.1.1%eth1 port 5001 connected with 192.168.1.4 port 45680 (MSS=1448) (burst-period=1.0000s) (trip-times) (sock=4) (peer 2.1.4-master) on 2021-10-26 20:25:29 (PDT)
> [ ID] Burst (start-end) Transfer Bandwidth XferTime (DC%) Reads=Dist NetPwr
> [ 1] 0.0001-0.0500 sec 40.1 KBytes 6.59 Mbits/sec 49.848 ms (5%) 12=12:0:0:0:0:0:0:0 0
> [ 1] 1.0002-1.0461 sec 40.0 KBytes 7.14 Mbits/sec 45.913 ms (4.6%) 10=10:0:0:0:0:0:0:0 0
> [ 1] 2.0002-2.0491 sec 40.0 KBytes 6.70 Mbits/sec 48.876 ms (4.9%) 11=11:0:0:0:0:0:0:0 0
> [ 1] 3.0002-3.0501 sec 40.0 KBytes 6.57 Mbits/sec 49.886 ms (5%) 10=10:0:0:0:0:0:0:0 0
> [ 1] 4.0002-4.0501 sec 40.0 KBytes 6.57 Mbits/sec 49.887 ms (5%) 10=10:0:0:0:0:0:0:0 0
> [ 1] 5.0002-5.0501 sec 40.0 KBytes 6.57 Mbits/sec 49.881 ms (5%) 10=10:0:0:0:0:0:0:0 0
> [ 1] 6.0002-6.0511 sec 40.0 KBytes 6.44 Mbits/sec 50.895 ms (5.1%) 10=10:0:0:0:0:0:0:0 0
> [ 1] 7.0002-7.0501 sec 40.0 KBytes 6.57 Mbits/sec 49.889 ms (5%) 10=10:0:0:0:0:0:0:0 0
> [ 1] 8.0002-8.0481 sec 40.0 KBytes 6.84 Mbits/sec 47.901 ms (4.8%) 11=11:0:0:0:0:0:0:0 0
> [ 1] 9.0002-9.0491 sec 40.0 KBytes 6.70 Mbits/sec 48.872 ms (4.9%) 10=10:0:0:0:0:0:0:0 0
> [ 1] 0.0000-10.0031 sec 400 KBytes 328 Kbits/sec 104=104:0:0:0:0:0:0:0
>
> Bob
>
> On Tue, Oct 26, 2021 at 6:12 PM Eric Dumazet <eric.dumazet at gmail.com <mailto:eric.dumazet at gmail.com>> wrote:
>
>
>
> On 10/26/21 4:38 PM, Christoph Paasch wrote:
> > Hi Bob,
> >
> >> On Oct 26, 2021, at 4:23 PM, Bob McMahon <bob.mcmahon at broadcom.com <mailto:bob.mcmahon at broadcom.com> <mailto:bob.mcmahon at broadcom.com <mailto:bob.mcmahon at broadcom.com>>> wrote:
> >> I'm confused. I don't see any blocking nor partial writes per the write() at the app level with TCP_NOTSENT_LOWAT set at 4 bytes. The burst is 40K, the write size is 4K and the watermark is 4 bytes. There are ten writes per burst.
> >
> > You are on Linux here, right?
> >
> > AFAICS, Linux will still accept whatever fits in an skb. And that is likely more than 4K (with GSO on by default).
>
> This (max payload per skb) can be tuned at the driver level, at least for experimental purposes or dedicated devices.
>
> ip link set dev eth0 gso_max_size 8000
>
> To fetch current values :
>
> ip -d link sh dev eth0
>
>
> >
> > However, do you go back to select() after each write() or do you loop over the write() calls?
> >
> >
> > Christoph
> >
> >> The S8 histograms are the times waiting on the select(). The first value is the bin number (multiplied by 100usec bin width) and second the bin count. The worst case time is at the end and is timestamped per unix epoch.
> >>
> >> The second run is over a controlled WiFi link where a 99.7% point of 4-8ms for a WiFi TX op arbitration win is in the ballpark. The first is 1G wired and is in the 600 usec range. (No media arbitration there.)
> >>
> >> [root at localhost iperf2-code]# src/iperf -c 10.19.87.9 --trip-times -i 1 -e --tcp-write-prefetch 4 -l 4K --burst-size=40K --histograms
> >> WARN: option of --burst-size without --burst-period defaults --burst-period to 1 second
> >> ------------------------------------------------------------
> >> Client connecting to 10.19.87.9, TCP port 5001 with pid 2124 (1 flows)
> >> Write buffer size: 4096 Byte
> >> Bursting: 40.0 KByte every 1.00 seconds
> >> TCP window size: 85.0 KByte (default)
> >> Event based writes (pending queue watermark at 4 bytes)
> >> Enabled select histograms bin-width=0.100 ms, bins=10000
> >> ------------------------------------------------------------
> >> [ 1] local 10.19.87.10%eth0 port 33166 connected with 10.19.87.9 port 5001 (MSS=1448) (prefetch=4) (trip-times) (sock=3) (ct=0.54 ms) on 2021-10-26 16:07:33 (PDT)
> >> [ ID] Interval Transfer Bandwidth Write/Err Rtry Cwnd/RTT NetPwr
> >> [ 1] 0.00-1.00 sec 40.1 KBytes 329 Kbits/sec 11/0 0 14K/5368 us 8
> >> [ 1] 0.00-1.00 sec S8-PDF: bin(w=100us):cnt(10)=1:1,2:5,3:2,4:1,11:1 (5.00/95.00/99.7%=1/11/11,Outliers=0,obl/obu=0/0) (1.089 ms/1635289653.928360)
> >> [ 1] 1.00-2.00 sec 40.0 KBytes 328 Kbits/sec 10/0 0 14K/569 us 72
> >> [ 1] 1.00-2.00 sec S8-PDF: bin(w=100us):cnt(10)=1:2,2:1,3:4,4:1,7:1,8:1 (5.00/95.00/99.7%=1/8/8,Outliers=0,obl/obu=0/0) (0.736 ms/1635289654.928088)
> >> [ 1] 2.00-3.00 sec 40.0 KBytes 328 Kbits/sec 10/0 0 14K/312 us 131
> >> [ 1] 2.00-3.00 sec S8-PDF: bin(w=100us):cnt(10)=1:3,2:2,3:2,5:2,6:1 (5.00/95.00/99.7%=1/6/6,Outliers=0,obl/obu=0/0) (0.548 ms/1635289655.927776)
> >> [ 1] 3.00-4.00 sec 40.0 KBytes 328 Kbits/sec 10/0 0 14K/302 us 136
> >> [ 1] 3.00-4.00 sec S8-PDF: bin(w=100us):cnt(10)=1:2,2:2,3:5,6:1 (5.00/95.00/99.7%=1/6/6,Outliers=0,obl/obu=0/0) (0.584 ms/1635289656.927814)
> >> [ 1] 4.00-5.00 sec 40.0 KBytes 328 Kbits/sec 10/0 0 14K/316 us 130
> >> [ 1] 4.00-5.00 sec S8-PDF: bin(w=100us):cnt(10)=1:3,3:2,4:2,5:2,6:1 (5.00/95.00/99.7%=1/6/6,Outliers=0,obl/obu=0/0) (0.572 ms/1635289657.927810)
> >> [ 1] 5.00-6.00 sec 40.0 KBytes 328 Kbits/sec 10/0 0 14K/253 us 162
> >> [ 1] 5.00-6.00 sec S8-PDF: bin(w=100us):cnt(10)=1:3,2:2,3:4,5:1 (5.00/95.00/99.7%=1/5/5,Outliers=0,obl/obu=0/0) (0.417 ms/1635289658.927630)
> >> [ 1] 6.00-7.00 sec 40.0 KBytes 328 Kbits/sec 10/0 0 14K/290 us 141
> >> [ 1] 6.00-7.00 sec S8-PDF: bin(w=100us):cnt(10)=1:3,3:3,4:3,6:1 (5.00/95.00/99.7%=1/6/6,Outliers=0,obl/obu=0/0) (0.573 ms/1635289659.927771)
> >> [ 1] 7.00-8.00 sec 40.0 KBytes 328 Kbits/sec 10/0 0 14K/359 us 114
> >> [ 1] 7.00-8.00 sec S8-PDF: bin(w=100us):cnt(10)=1:2,3:4,4:3,6:1 (5.00/95.00/99.7%=1/6/6,Outliers=0,obl/obu=0/0) (0.570 ms/1635289660.927753)
> >> [ 1] 8.00-9.00 sec 40.0 KBytes 328 Kbits/sec 10/0 0 14K/349 us 117
> >> [ 1] 8.00-9.00 sec S8-PDF: bin(w=100us):cnt(10)=1:3,3:5,4:1,7:1 (5.00/95.00/99.7%=1/7/7,Outliers=0,obl/obu=0/0) (0.608 ms/1635289661.927843)
> >> [ 1] 9.00-10.00 sec 40.0 KBytes 328 Kbits/sec 10/0 0 14K/347 us 118
> >> [ 1] 9.00-10.00 sec S8-PDF: bin(w=100us):cnt(10)=1:3,2:1,3:5,8:1 (5.00/95.00/99.7%=1/8/8,Outliers=0,obl/obu=0/0) (0.725 ms/1635289662.927861)
> >> [ 1] 0.00-10.01 sec 400 KBytes 327 Kbits/sec 102/0 0 14K/1519 us 27
> >> [ 1] 0.00-10.01 sec S8(f)-PDF: bin(w=100us):cnt(100)=1:25,2:13,3:36,4:11,5:5,6:5,7:2,8:2,11:1 (5.00/95.00/99.7%=1/7/11,Outliers=0,obl/obu=0/0) (1.089 ms/1635289653.928360)
> >>
> >> [root at localhost iperf2-code]# src/iperf -c 192.168.1.1 --trip-times -i 1 -e --tcp-write-prefetch 4 -l 4K --burst-size=40K --histograms
> >> WARN: option of --burst-size without --burst-period defaults --burst-period to 1 second
> >> ------------------------------------------------------------
> >> Client connecting to 192.168.1.1, TCP port 5001 with pid 2131 (1 flows)
> >> Write buffer size: 4096 Byte
> >> Bursting: 40.0 KByte every 1.00 seconds
> >> TCP window size: 85.0 KByte (default)
> >> Event based writes (pending queue watermark at 4 bytes)
> >> Enabled select histograms bin-width=0.100 ms, bins=10000
> >> ------------------------------------------------------------
> >> [ 1] local 192.168.1.4%eth1 port 45518 connected with 192.168.1.1 port 5001 (MSS=1448) (prefetch=4) (trip-times) (sock=3) (ct=5.48 ms) on 2021-10-26 16:07:56 (PDT)
> >> [ ID] Interval Transfer Bandwidth Write/Err Rtry Cwnd/RTT NetPwr
> >> [ 1] 0.00-1.00 sec 40.1 KBytes 329 Kbits/sec 11/0 0 14K/10339 us 4
> >> [ 1] 0.00-1.00 sec S8-PDF: bin(w=100us):cnt(10)=1:1,40:1,47:1,49:2,50:3,51:1,60:1 (5.00/95.00/99.7%=1/60/60,Outliers=0,obl/obu=0/0) (5.990 ms/1635289676.802143)
> >> [ 1] 1.00-2.00 sec 40.0 KBytes 328 Kbits/sec 10/0 0 14K/4853 us 8
> >> [ 1] 1.00-2.00 sec S8-PDF: bin(w=100us):cnt(10)=1:2,38:1,39:1,44:1,45:1,49:1,51:1,52:1,60:1 (5.00/95.00/99.7%=1/60/60,Outliers=0,obl/obu=0/0) (5.937 ms/1635289677.802274)
> >> [ 1] 2.00-3.00 sec 40.0 KBytes 328 Kbits/sec 10/0 0 14K/4991 us 8
> >> [ 1] 2.00-3.00 sec S8-PDF: bin(w=100us):cnt(10)=1:2,48:1,49:2,50:2,51:1,60:1,64:1 (5.00/95.00/99.7%=1/64/64,Outliers=0,obl/obu=0/0) (6.307 ms/1635289678.794326)
> >> [ 1] 3.00-4.00 sec 40.0 KBytes 328 Kbits/sec 10/0 0 14K/4610 us 9
> >> [ 1] 3.00-4.00 sec S8-PDF: bin(w=100us):cnt(10)=1:2,49:3,50:3,56:1,64:1 (5.00/95.00/99.7%=1/64/64,Outliers=0,obl/obu=0/0) (6.362 ms/1635289679.794335)
> >> [ 1] 4.00-5.00 sec 40.0 KBytes 328 Kbits/sec 10/0 0 14K/5028 us 8
> >> [ 1] 4.00-5.00 sec S8-PDF: bin(w=100us):cnt(10)=1:2,49:6,59:1,64:1 (5.00/95.00/99.7%=1/64/64,Outliers=0,obl/obu=0/0) (6.367 ms/1635289680.794399)
> >> [ 1] 5.00-6.00 sec 40.0 KBytes 328 Kbits/sec 10/0 0 14K/5113 us 8
> >> [ 1] 5.00-6.00 sec S8-PDF: bin(w=100us):cnt(10)=1:2,49:3,50:2,58:1,60:1,65:1 (5.00/95.00/99.7%=1/65/65,Outliers=0,obl/obu=0/0) (6.442 ms/1635289681.794392)
> >> [ 1] 6.00-7.00 sec 40.0 KBytes 328 Kbits/sec 10/0 0 14K/5054 us 8
> >> [ 1] 6.00-7.00 sec S8-PDF: bin(w=100us):cnt(10)=1:2,39:1,49:3,51:1,60:2,64:1 (5.00/95.00/99.7%=1/64/64,Outliers=0,obl/obu=0/0) (6.374 ms/1635289682.794335)
> >> [ 1] 7.00-8.00 sec 40.0 KBytes 328 Kbits/sec 10/0 0 14K/5138 us 8
> >> [ 1] 7.00-8.00 sec S8-PDF: bin(w=100us):cnt(10)=1:2,39:2,40:1,49:2,50:1,60:1,64:1 (5.00/95.00/99.7%=1/64/64,Outliers=0,obl/obu=0/0) (6.396 ms/1635289683.794338)
> >> [ 1] 8.00-9.00 sec 40.0 KBytes 328 Kbits/sec 10/0 0 14K/5329 us 8
> >> [ 1] 8.00-9.00 sec S8-PDF: bin(w=100us):cnt(10)=1:2,38:1,45:2,49:1,50:3,63:1 (5.00/95.00/99.7%=1/63/63,Outliers=0,obl/obu=0/0) (6.292 ms/1635289684.794262)
> >> [ 1] 9.00-10.00 sec 40.0 KBytes 328 Kbits/sec 10/0 0 14K/5329 us 8
> >> [ 1] 9.00-10.00 sec S8-PDF: bin(w=100us):cnt(10)=1:2,39:1,49:3,50:3,84:1 (5.00/95.00/99.7%=1/84/84,Outliers=0,obl/obu=0/0) (8.306 ms/1635289685.796315)
> >> [ 1] 0.00-10.01 sec 400 KBytes 327 Kbits/sec 102/0 0 14K/6331 us 6
> >> [ 1] 0.00-10.01 sec S8(f)-PDF: bin(w=100us):cnt(100)=1:19,38:2,39:5,40:2,44:1,45:3,47:1,48:1,49:26,50:17,51:4,52:1,56:1,58:1,59:1,60:7,63:1,64:5,65:1,84:1 (5.00/95.00/99.7%=1/64/84,Outliers=0,obl/obu=0/0) (8.306 ms/1635289685.796315)
> >>
> >> Bob
> >>
> >> On Tue, Oct 26, 2021 at 11:45 AM Christoph Paasch <cpaasch at apple.com <mailto:cpaasch at apple.com> <mailto:cpaasch at apple.com <mailto:cpaasch at apple.com>>> wrote:
> >>
> >> Hello,
> >>
> >> > On Oct 25, 2021, at 9:24 PM, Eric Dumazet <eric.dumazet at gmail.com <mailto:eric.dumazet at gmail.com> <mailto:eric.dumazet at gmail.com <mailto:eric.dumazet at gmail.com>>> wrote:
> >> >
> >> >
> >> >
> >> > On 10/25/21 8:11 PM, Stuart Cheshire via Bloat wrote:
> >> >> On 21 Oct 2021, at 17:51, Bob McMahon via Make-wifi-fast <make-wifi-fast at lists.bufferbloat.net <mailto:make-wifi-fast at lists.bufferbloat.net> <mailto:make-wifi-fast at lists.bufferbloat.net <mailto:make-wifi-fast at lists.bufferbloat.net>>> wrote:
> >> >>
> >> >>> Hi All,
> >> >>>
> >> >>> Sorry for the spam. I'm trying to support a meaningful TCP message latency w/iperf 2 from the sender side w/o requiring e2e clock synchronization. I thought I'd try to use the TCP_NOTSENT_LOWAT event to help with this. It seems that this event goes off when the bytes are in flight vs have reached the destination network stack. If that's the case, then iperf 2 client (sender) may be able to produce the message latency by adding the drain time (write start to TCP_NOTSENT_LOWAT) and the sampled RTT.
> >> >>>
> >> >>> Does this seem reasonable?
> >> >>
> >> >> I’m not 100% sure what you’re asking, but I will try to help.
> >> >>
> >> >> When you set TCP_NOTSENT_LOWAT, the TCP implementation won’t report your endpoint as writable (e.g., via kqueue or epoll) until less than that threshold of data remains unsent. It won’t stop you writing more bytes if you want to, up to the socket send buffer size, but it won’t *ask* you for more data until the TCP_NOTSENT_LOWAT threshold is reached.
> >> >
> >> >
> >> > When I implemented TCP_NOTSENT_LOWAT back in 2013 [1], I made sure that sendmsg() would actually
> >> > stop feeding more bytes in TCP transmit queue if the current amount of unsent bytes
> >> > was above the threshold.
> >> >
> >> > So it looks like Apple implementation is different, based on your description ?
> >>
> >> Yes, TCP_NOTSENT_LOWAT only impacts the wakeup on iOS/macOS/...
> >>
> >> An app can still fill the send-buffer if it does a sendmsg() with a large buffer or does repeated calls to sendmsg().
> >>
> >> Fur Apple, the goal of TCP_NOTSENT_LOWAT was to allow an app to quickly change the data it "scheduled" to send. And thus allow the app to write the smallest "logical unit" it has. If that unit is 512KB large, the app is allowed to send that.
> >> For example, in case of video-streaming one may want to skip ahead in the video. In that case the app still needs to transmit the remaining parts of the previous frame anyways, before it can send the new video frame.
> >> That's the reason why the Apple implementation allows one to write more than just the lowat threshold.
> >>
> >>
> >> That being said, I do think that Linux's way allows for an easier API because the app does not need to be careful at how much data it sends after an epoll/kqueue wakeup. So, the latency-benefits will be easier to get.
> >>
> >>
> >> Christoph
> >>
> >>
> >>
> >> > [1] https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git/commit/?id=c9bee3b7fdecb0c1d070c7b54113b3bdfb9a3d36 <https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git/commit/?id=c9bee3b7fdecb0c1d070c7b54113b3bdfb9a3d36> <https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git/commit/?id=c9bee3b7fdecb0c1d070c7b54113b3bdfb9a3d36 <https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git/commit/?id=c9bee3b7fdecb0c1d070c7b54113b3bdfb9a3d36>>
> >> >
> >> > netperf does not use epoll(), but rather a loop over sendmsg().
> >> >
> >> > One of the point of TCP_NOTSENT_LOWAT for Google was to be able to considerably increase
> >> > max number of bytes in transmit queues (3rd column of /proc/sys/net/ipv4/tcp_wmem)
> >> > by 10x, allowing for autotune to increase BDP for big RTT flows, this without
> >> > increasing memory needs for flows with small RTT.
> >> >
> >> > In other words, the TCP implementation attempts to keep BDP bytes in flight + TCP_NOTSENT_LOWAT bytes buffered and ready to go. The BDP of bytes in flight is necessary to fill the network pipe and get good throughput. The TCP_NOTSENT_LOWAT of bytes buffered and ready to go is provided to give the source software some advance notice that the TCP implementation will soon be looking for more bytes to send, so that the buffer doesn’t run dry, thereby lowering throughput. (The old SO_SNDBUF option conflates both “bytes in flight” and “bytes buffered and ready to go” into the same number.)
> >> >>
> >> >> If you wait for the TCP_NOTSENT_LOWAT notification, write a chunk of n bytes of data, and then wait for the next TCP_NOTSENT_LOWAT notification, that will tell you roughly how long it took n bytes to depart the machine. You won’t know why, though. The bytes could depart the machine in response for acks indicating that the same number of bytes have been accepted at the receiver. But the bytes can also depart the machine because CWND is growing. Of course, both of those things are usually happening at the same time.
> >> >>
> >> >> How to use TCP_NOTSENT_LOWAT is explained in this video:
> >> >>
> >> >> <https://developer.apple.com/videos/play/wwdc2015/719/?time=2199 <https://developer.apple.com/videos/play/wwdc2015/719/?time=2199> <https://developer.apple.com/videos/play/wwdc2015/719/?time=2199 <https://developer.apple.com/videos/play/wwdc2015/719/?time=2199>>>
> >> >>
> >> >> Later in the same video is a two-minute demo (time offset 42:00 to time offset 44:00) showing a “before and after” demo illustrating the dramatic difference this makes for screen sharing responsiveness.
> >> >>
> >> >> <https://developer.apple.com/videos/play/wwdc2015/719/?time=2520 <https://developer.apple.com/videos/play/wwdc2015/719/?time=2520> <https://developer.apple.com/videos/play/wwdc2015/719/?time=2520 <https://developer.apple.com/videos/play/wwdc2015/719/?time=2520>>>
> >> >>
> >> >> Stuart Cheshire
> >> >> _______________________________________________
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> >> >> https://lists.bufferbloat.net/listinfo/bloat <https://lists.bufferbloat.net/listinfo/bloat> <https://lists.bufferbloat.net/listinfo/bloat <https://lists.bufferbloat.net/listinfo/bloat>>
> >> >>
> >> > _______________________________________________
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> >> > Bloat at lists.bufferbloat.net <mailto:Bloat at lists.bufferbloat.net> <mailto:Bloat at lists.bufferbloat.net <mailto:Bloat at lists.bufferbloat.net>>
> >> > https://lists.bufferbloat.net/listinfo/bloat <https://lists.bufferbloat.net/listinfo/bloat> <https://lists.bufferbloat.net/listinfo/bloat <https://lists.bufferbloat.net/listinfo/bloat>>
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