Greetings:
Welcome to v2, see changelog below.
This series introduces a new mechanism, IRQ suspension, which allows
network applications using epoll to mask IRQs during periods of high
traffic while also reducing tail latency (compared to existing
mechanisms, see below) during periods of low traffic. In doing so, this
balances CPU consumption with network processing efficiency.
Martin Karsten (CC'd) and I have been collaborating on this series for
several months and have appreciated the feedback from the community on
our RFC [1]. We've updated the cover letter and kernel documentation in
an attempt to more clearly explain how this mechanism works, how
applications can use it, and how it compares to existing mechanisms in
the kernel. We've added an additional test case, 'fullbusy', achieved by
modifying libevent for comparison. See below for a detailed description,
link to the patch, and test results.
I briefly mentioned this idea at netdev conf 2024 (for those who were
there) and Martin described this idea in an earlier paper presented at
Sigmetrics 2024 [2].
~ The short explanation (TL;DR)
We propose adding a new napi config parameter: irq_suspend_timeout to
help balance CPU usage and network processing efficiency when using IRQ
deferral and napi busy poll.
If this parameter is set to a non-zero value *and* a user application
has enabled preferred busy poll on a busy poll context (via the
EPIOCSPARAMS ioctl introduced in commit 18e2bf0edf4d ("eventpoll: Add
epoll ioctl for epoll_params")), then application calls to epoll_wait
for that context will cause device IRQs and softirq processing to be
suspended as long as epoll_wait successfully retrieves data from the
NAPI. Each time data is retrieved, the irq_suspend_timeout is deferred.
If/when network traffic subsides and epoll_wait returns no data, IRQ
suspension is immediately reverted back to the existing
napi_defer_hard_irqs and gro_flush_timeout mechanism which was
introduced in commit 6f8b12d661d0 ("net: napi: add hard irqs deferral
feature")).
The irq_suspend_timeout serves as a safety mechanism. If userland takes
a long time processing data, irq_suspend_timeout will fire and restart
normal NAPI processing.
For a more in depth explanation, please continue reading.
~ Comparison with existing mechanisms
Interrupt mitigation can be accomplished in napi software, by setting
napi_defer_hard_irqs and gro_flush_timeout, or via interrupt coalescing
in the NIC. This can be quite efficient, but in both cases, a fixed
timeout (or packet count) needs to be configured. However, a fixed
timeout cannot effectively support both low- and high-load situations:
At low load, an application typically processes a few requests and then
waits to receive more input data. In this scenario, a large timeout will
cause unnecessary latency.
At high load, an application typically processes many requests before
being ready to receive more input data. In this case, a small timeout
will likely fire prematurely and trigger irq/softirq processing, which
interferes with the application's execution. This causes overhead, most
likely due to cache contention.
While NICs attempt to provide adaptive interrupt coalescing schemes,
these cannot properly take into account application-level processing.
An alternative packet delivery mechanism is busy-polling, which results
in perfect alignment of application processing and network polling. It
delivers optimal performance (throughput and latency), but results in
100% cpu utilization and is thus inefficient for below-capacity
workloads.
We propose to add a new packet delivery mode that properly alternates
between busy polling and interrupt-based delivery depending on busy and
idle periods of the application. During a busy period, the system
operates in busy-polling mode, which avoids interference. During an idle
period, the system falls back to interrupt deferral, but with a small
timeout to avoid excessive latencies. This delivery mode can also be
viewed as an extension of basic interrupt deferral, but alternating
between a small and a very large timeout.
This delivery mode is efficient, because it avoids softirq execution
interfering with application processing during busy periods. It can be
used with blocking epoll_wait to conserve cpu cycles during idle
periods. The effect of alternating between busy and idle periods is that
performance (throughput and latency) is very close to full busy polling,
while cpu utilization is lower and very close to interrupt mitigation.
~ Usage details
IRQ suspension is introduced via a per-NAPI configuration parameter that
controls the maximum time that IRQs can be suspended.
Here's how it is intended to work:
- The user application (or system administrator) uses the netdev-genl
netlink interface to set the pre-existing napi_defer_hard_irqs and
gro_flush_timeout NAPI config parameters to enable IRQ deferral.
- The user application (or system administrator) sets the proposed
irq_suspend_timeout parameter via the netdev-genl netlink interface
to a larger value than gro_flush_timeout to enable IRQ suspension.
- The user application issues the existing epoll ioctl to set the
prefer_busy_poll flag on the epoll context.
- The user application then calls epoll_wait to busy poll for network
events, as it normally would.
- If epoll_wait returns events to userland, IRQs are suspended for the
duration of irq_suspend_timeout.
- If epoll_wait finds no events and the thread is about to go to
sleep, IRQ handling using napi_defer_hard_irqs and gro_flush_timeout
is resumed.
As long as epoll_wait is retrieving events, IRQs (and softirq
processing) for the NAPI being polled remain disabled. When network
traffic reduces, eventually a busy poll loop in the kernel will retrieve
no data. When this occurs, regular IRQ deferral using gro_flush_timeout
for the polled NAPI is re-enabled.
Unless IRQ suspension is continued by subsequent calls to epoll_wait, it
automatically times out after the irq_suspend_timeout timer expires.
Regular deferral is also immediately re-enabled when the epoll context
is destroyed.
~ Usage scenario
The target scenario for IRQ suspension as packet delivery mode is a
system that runs a dominant application with substantial network I/O.
The target application can be configured to receive input data up to a
certain batch size (via epoll_wait maxevents parameter) and this batch
size determines the worst-case latency that application requests might
experience. Because packet delivery is suspended during the target
application's processing, the batch size also determines the worst-case
latency of concurrent applications using the same RX queue(s).
gro_flush_timeout should be set as small as possible, but large enough to
make sure that a single request is likely not being interfered with.
irq_suspend_timeout is largely a safety mechanism against misbehaving
applications. It should be set large enough to cover the processing of an
entire application batch, i.e., the factor between gro_flush_timeout and
irq_suspend_timeout should roughly correspond to the maximum batch size
that the target application would process in one go.
~ Design rationale
The implementation of the IRQ suspension mechanism very nicely dovetails
with the existing mechanism for IRQ deferral when preferred busy poll is
enabled (introduced in commit 7fd3253a7de6 ("net: Introduce preferred
busy-polling"), see that commit message for more details).
While it would be possible to inject the suspend timeout via
the existing epoll ioctl, it is more natural to avoid this path for one
main reason:
An epoll context is linked to NAPI IDs as file descriptors are added;
this means any epoll context might suddenly be associated with a
different net_device if the application were to replace all existing
fds with fds from a different device. In this case, the scope of the
suspend timeout becomes unclear and many edge cases for both the user
application and the kernel are introduced
Only a single iteration through napi busy polling is needed for this
mechanism to work effectively. Since an important objective for this
mechanism is preserving cpu cycles, exactly one iteration of the napi
busy loop is invoked when busy_poll_usecs is set to 0.
~ Important call outs in the implementation
- Enabling per epoll-context preferred busy poll will now effectively
lead to a nonblocking iteration through napi_busy_loop, even when
busy_poll_usecs is 0. See patch 4.
- Patches apply cleanly on commit 160a810b2a85 ("net: vxlan: update
the document for vxlan_snoop()").
~ Benchmark configs & descriptions
The changes were benchmarked with memcached [3] using the benchmarking
tool mutilate [4].
To facilitate benchmarking, a small patch [5] was applied to memcached
1.6.29 to allow setting per-epoll context preferred busy poll and other
settings via environment variables. Another small patch [6] was applied
to libevent to enable full busy-polling.
Multiple scenarios were benchmarked as described below and the scripts
used for producing these results can be found on github [7] (note: all
scenarios use NAPI-based traffic splitting via SO_INCOMING_ID by passing
-N to memcached):
- base:
- no other options enabled
- deferX:
- set defer_hard_irqs to 100
- set gro_flush_timeout to X,000
- napibusy:
- set defer_hard_irqs to 100
- set gro_flush_timeout to 200,000
- enable busy poll via the existing ioctl (busy_poll_usecs = 64,
busy_poll_budget = 64, prefer_busy_poll = true)
- fullbusy:
- set defer_hard_irqs to 100
- set gro_flush_timeout to 5,000,000
- enable busy poll via the existing ioctl (busy_poll_usecs = 1000,
busy_poll_budget = 64, prefer_busy_poll = true)
- change memcached's nonblocking epoll_wait invocation (via
libevent) to using a 1 ms timeout
- suspendX:
- set defer_hard_irqs to 100
- set gro_flush_timeout to X,000
- set irq_suspend_timeout to 20,000,000
- enable busy poll via the existing ioctl (busy_poll_usecs = 0,
busy_poll_budget = 64, prefer_busy_poll = true)
~ Benchmark results
Tested on:
Single socket AMD EPYC 7662 64-Core Processor
Hyperthreading disabled
4 NUMA Zones (NPS=4)
16 CPUs per NUMA zone (64 cores total)
2 x Dual port 100gbps Mellanox Technologies ConnectX-5 Ex EN NIC
The test machine is configured such that a single interface has 8 RX
queues. The queues' IRQs and memcached are pinned to CPUs that are
NUMA-local to the interface which is under test. The NIC's interrupt
coalescing configuration is left at boot-time defaults.
Results:
Results are shown below. The mechanism added by this series is
represented by the 'suspend' cases. Data presented shows a summary over
at least 10 runs of each test case [8] using the scripts on github [7].
For latency, the median is shown. For throughput and CPU utilization,
the average is shown.
The results also include cycles-per-query (cpq) and
instruction-per-query (ipq) metrics, following the methodology proposed
in [2], to augment the CPU utilization numbers, which could be skewed
due to frequency scaling. We find that this does not appear to be the
case as CPU utilization and low-level metrics show similar trends.
These results were captured using the scripts on github [7] to
illustrate how this approach compares with other pre-existing
mechanisms. This data is not to be interpreted as scientific data
captured in a fully isolated lab setting, but instead as best effort,
illustrative information comparing and contrasting tradeoffs.
The absolute QPS results are higher than our previous submission, but
the relative differences between variants are equivalent. Because the
patches have been rebased on 6.12, several factors have likely
influenced the overall performance. Most importantly, we had to switch
to a new set of basic kernel options, which has likely altered the
baseline performance. Because the overall comparison of variants still
holds, we have not attempted to recreate the exact set of kernel options
from the previous submission.
Compare:
- Throughput (MAX) and latencies of base vs suspend.
- CPU usage of napibusy and fullbusy during lower load (200K, 400K for
example) vs suspend.
- Latency of the defer variants vs suspend as timeout and load
increases.
The overall takeaway is that the suspend variants provide a superior
combination of high throughput, low latency, and low cpu utilization
compared to all other variants. Each of the suspend variants works very
well, but some fine-tuning between latency and cpu utilization is still
possible by tuning the small timeout (gro_flush_timeout).
Note: we've reorganized the results to make comparison among testcases
with the same load easier.
testcase load qps avglat 95%lat 99%lat cpu cpq ipq
base 200K 200024 127 254 458 25 12748 11289
defer10 200K 199991 64 128 166 27 18763 16574
defer20 200K 199986 72 135 178 25 15405 14173
defer50 200K 200025 91 149 198 23 12275 12203
defer200 200K 199996 182 266 326 18 8595 9183
fullbusy 200K 200040 58 123 167 100 43641 23145
napibusy 200K 200009 115 244 299 56 24797 24693
suspend10 200K 200005 63 128 167 32 19559 17240
suspend20 200K 199952 69 132 170 29 16324 14838
suspend50 200K 200019 84 144 189 26 13106 12516
suspend200 200K 199978 168 264 326 20 9331 9643
testcase load qps avglat 95%lat 99%lat cpu cpq ipq
base 400K 400017 157 292 762 39 9287 9325
defer10 400K 400033 71 141 204 53 13950 12943
defer20 400K 399935 79 150 212 47 12027 11673
defer50 400K 399888 101 171 231 39 9556 9921
defer200 400K 399993 200 287 358 32 7428 8576
fullbusy 400K 400018 63 132 203 100 21827 16062
napibusy 400K 399970 89 230 292 83 18156 16508
suspend10 400K 400061 69 139 202 54 13576 13057
suspend20 400K 399988 73 144 206 49 11930 11773
suspend50 400K 399975 88 161 218 42 9996 10270
suspend200 400K 399954 172 276 353 34 7847 8713
testcase load qps avglat 95%lat 99%lat cpu cpq ipq
base 600K 600031 166 289 631 61 9188 8787
defer10 600K 599967 85 167 262 75 11833 10947
defer20 600K 599888 89 165 243 66 10513 10362
defer50 600K 600072 109 185 253 55 8664 9190
defer200 600K 599951 222 315 393 45 6892 8213
fullbusy 600K 600041 69 145 227 100 14549 13936
napibusy 600K 599980 79 188 280 96 13927 14155
suspend10 600K 600028 78 159 267 69 10877 11032
suspend20 600K 600026 81 159 254 64 9922 10320
suspend50 600K 600007 96 178 258 57 8681 9331
suspend200 600K 599964 177 295 369 47 7115 8366
testcase load qps avglat 95%lat 99%lat cpu cpq ipq
base 800K 800034 198 329 698 84 9366 8338
defer10 800K 799718 243 642 1457 95 10532 9007
defer20 800K 800009 132 245 399 89 9956 8979
defer50 800K 800024 136 228 378 80 9002 8598
defer200 800K 799965 255 362 473 66 7481 8147
fullbusy 800K 799927 78 157 253 100 10915 12533
napibusy 800K 799870 81 173 273 99 10826 12532
suspend10 800K 799991 84 167 269 83 9380 9802
suspend20 800K 799979 90 172 290 78 8765 9404
suspend50 800K 800031 106 191 307 71 7945 8805
suspend200 800K 799905 182 307 411 62 6985 8242
testcase load qps avglat 95%lat 99%lat cpu cpq ipq
base 1000K 919543 3805 6390 14229 98 9324 7978
defer10 1000K 850751 4574 7382 15370 99 10218 8470
defer20 1000K 890296 4736 6862 14858 99 9708 8277
defer50 1000K 932694 3463 6180 13251 97 9148 8053
defer200 1000K 951311 3524 6052 13599 96 8875 7845
fullbusy 1000K 1000011 90 181 278 100 8731 10686
napibusy 1000K 1000050 93 184 280 100 8721 10547
suspend10 1000K 999962 101 193 306 92 8138 8980
suspend20 1000K 1000030 103 191 324 88 7844 8763
suspend50 1000K 1000001 114 202 320 83 7396 8431
suspend200 1000K 999965 185 314 428 76 6733 8072
testcase load qps avglat 95%lat 99%lat cpu cpq ipq
base MAX 1005592 4651 6594 14979 100 8679 7918
defer10 MAX 928204 5106 7286 15199 100 9398 8380
defer20 MAX 984663 4774 6518 14920 100 8861 8063
defer50 MAX 1044099 4431 6368 14652 100 8350 7948
defer200 MAX 1040451 4423 6610 14674 100 8380 7931
fullbusy MAX 1236608 3715 3987 12805 100 7051 7936
napibusy MAX 1077516 4345 10155 15957 100 8080 7842
suspend10 MAX 1218344 3760 3990 12585 100 7150 7935
suspend20 MAX 1220056 3752 4053 12602 100 7150 7961
suspend50 MAX 1213666 3791 4103 12919 100 7183 7959
suspend200 MAX 1217411 3768 3988 12863 100 7161 7954
~ FAQ
- Can the new timeout value be threaded through the new epoll ioctl ?
Only with difficulty. The epoll ioctl sets options on an epoll
context and the NAPI ID associated with an epoll context can change
based on what file descriptors a user app adds to the epoll context.
This would introduce complexity in the API from the user perspective
and also complexity in the kernel.
- Can irq suspend be built by combining NIC coalescing and
gro_flush_timeout ?
No. The problem is that the long timeout must engage if and only if
prefer-busy is active.
When using NIC coalescing for the short timeout (without
napi_defer_hard_irqs/gro_flush_timeout), an interrupt after an idle
period will trigger softirq, which will run napi polling. At this
point, prefer-busy is not active, so NIC interrupts would be
re-enabled. Then it is not possible for the longer timeout to
interject to switch control back to polling. In other words, only by
using the software timer for the short timeout, it is possible to
extend the timeout without having to reprogram the NIC timer or
reach down directly and disable interrupts.
Using gro_flush_timeout for the long timeout also has problems, for
the same underlying reason. In the current napi implementation,
gro_flush_timeout is not tied to prefer-busy. We'd either have to
change that and in the process modify the existing deferral
mechanism, or introduce a state variable to determine whether
gro_flush_timeout is used as long timeout for irq suspend or whether
it is used for its default purpose. In an earlier version, we did
try something similar to the latter and made it work, but it ends up
being a lot more convoluted than our current proposal.
- Isn't it already possible to combine busy looping with irq deferral?
Yes, in fact enabling irq deferral via napi_defer_hard_irqs and
gro_flush_timeout is a precondition for prefer_busy_poll to have an
effect. If the application also uses a tight busy loop with
essentially nonblocking epoll_wait (accomplished with a very short
timeout parameter), this is the fullbusy case shown in the results.
An application using blocking epoll_wait is shown as the napibusy
case in the result. It's a hybrid approach that provides limited
latency benefits compared to the base case and plain irq deferral,
but not as good as fullbusy or suspend.
~ Special thanks
Several people were involved in earlier stages of the development of this
mechanism whom we'd like to thank:
- Peter Cai (CC'd), for the initial kernel patch and his contributions
to the paper.
- Mohammadamin Shafie (CC'd), for testing various versions of the kernel
patch and providing helpful feedback.
Thanks,
Martin and Joe
[1]: https://lore.kernel.org/netdev/20240812125717.413108-1-jdamato@xxxxxxxxxx/
[2]: https://doi.org/10.1145/3626780
[3]: https://github.com/memcached/memcached/blob/master/doc/napi_ids.txt
[4]: https://github.com/leverich/mutilate
[5]: https://raw.githubusercontent.com/martinkarsten/irqsuspend/main/patches/memcached.patch
[6]: https://raw.githubusercontent.com/martinkarsten/irqsuspend/main/patches/libevent.patch
[7]: https://github.com/martinkarsten/irqsuspend
[8]: https://github.com/martinkarsten/irqsuspend/tree/main/results
v2:
- Cover letter updated, including a re-run of test data.
- Patch 1 rewritten to use netdev-genl instead of sysfs.
- Patch 3 updated with a comment added to napi_resume_irqs.
- Patch 4 rebased to apply now that commit b9ca079dd6b0 ("eventpoll:
Annotate data-race of busy_poll_usecs") has been picked up from VFS.
- Patch 6 updated the kernel documentation.
rfc -> v1: https://lore.kernel.org/netdev/20240823173103.94978-1-jdamato@xxxxxxxxxx/
- Cover letter updated to include more details.
- Patch 1 updated to remove the documentation added. This was moved to
patch 6 with the rest of the docs (see below).
- Patch 5 updated to fix an error uncovered by the kernel build robot.
See patch 5's changelog for more details.
- Patch 6 added which updates kernel documentation.
Joe Damato (1):
docs: networking: Describe irq suspension
Martin Karsten (5):
net: Add napi_struct parameter irq_suspend_timeout
net: Suspend softirq when prefer_busy_poll is set
net: Add control functions for irq suspension
eventpoll: Trigger napi_busy_loop, if prefer_busy_poll is set
eventpoll: Control irq suspension for prefer_busy_poll
Documentation/netlink/specs/netdev.yaml | 7 ++
Documentation/networking/napi.rst | 132 +++++++++++++++++++++++-
fs/eventpoll.c | 35 ++++++-
include/linux/netdevice.h | 2 +
include/net/busy_poll.h | 3 +
include/uapi/linux/netdev.h | 1 +
net/core/dev.c | 58 ++++++++++-
net/core/dev.h | 25 +++++
net/core/netdev-genl-gen.c | 5 +-
net/core/netdev-genl.c | 12 +++
tools/include/uapi/linux/netdev.h | 1 +
11 files changed, 271 insertions(+), 10 deletions(-)
base-commit: 160a810b2a8588187ec2b1536d0355c0aab8981c
--
2.25.1
