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vhost_user_block: Implement optional static polling

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Actively polling the virtqueue significantly reduces the latency of
each I/O operation, at the expense of using more CPU time. This
features is specially useful when using low-latency devices (SSD,
NVMe) as the backend.

This change implements static polling. When a request arrives after
being idle, vhost_user_block will keep checking the virtqueue for new
requests, until POLL_QUEUE_US (50us) has passed without finding one.

POLL_QUEUE_US is defined to be 50us, based on the current latency of
enterprise SSDs (< 30us) and the overhead of the emulation.

This feature is enabled by default, and can be disabled by using the
"poll_queue" parameter of "block-backend".

This is a test using null_blk as a backend for the image, with the
following parameters:

 - null_blk gb=20 nr_devices=1 irqmode=2 completion_nsec=0 no_sched=1

With "poll_queue=false":

fio --ioengine=sync --bs=4k --rw randread --name randread --direct=1
--filename=/dev/vdb --time_based --runtime=10

randread: (g=0): rw=randread, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=sync, iodepth=1
fio-3.14
Starting 1 process
Jobs: 1 (f=1): [r(1)][100.0%][r=169MiB/s][r=43.2k IOPS][eta 00m:00s]
randread: (groupid=0, jobs=1): err= 0: pid=433: Tue Feb 18 11:12:59 2020
  read: IOPS=43.2k, BW=169MiB/s (177MB/s)(1688MiB/10001msec)
    clat (usec): min=17, max=836, avg=21.64, stdev= 3.81
     lat (usec): min=17, max=836, avg=21.77, stdev= 3.81
    clat percentiles (nsec):
     |  1.00th=[19328],  5.00th=[19840], 10.00th=[20352], 20.00th=[21120],
     | 30.00th=[21376], 40.00th=[21376], 50.00th=[21376], 60.00th=[21632],
     | 70.00th=[21632], 80.00th=[21888], 90.00th=[22144], 95.00th=[22912],
     | 99.00th=[28544], 99.50th=[30336], 99.90th=[39168], 99.95th=[42752],
     | 99.99th=[71168]
   bw (  KiB/s): min=168440, max=188496, per=100.00%, avg=172912.00, stdev=3975.63, samples=19
   iops        : min=42110, max=47124, avg=43228.00, stdev=993.91, samples=19
  lat (usec)   : 20=5.90%, 50=94.08%, 100=0.02%, 250=0.01%, 500=0.01%
  lat (usec)   : 750=0.01%, 1000=0.01%
  cpu          : usr=10.35%, sys=25.82%, ctx=432417, majf=0, minf=10
  IO depths    : 1=100.0%, 2=0.0%, 4=0.0%, 8=0.0%, 16=0.0%, 32=0.0%, >=64=0.0%
     submit    : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
     complete  : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
     issued rwts: total=432220,0,0,0 short=0,0,0,0 dropped=0,0,0,0
     latency   : target=0, window=0, percentile=100.00%, depth=1

Run status group 0 (all jobs):
   READ: bw=169MiB/s (177MB/s), 169MiB/s-169MiB/s (177MB/s-177MB/s), io=1688MiB (1770MB), run=10001-10001msec

Disk stats (read/write):
  vdb: ios=427867/0, merge=0/0, ticks=7346/0, in_queue=0, util=99.04%

With "poll_queue=true" (default):

fio --ioengine=sync --bs=4k --rw randread --name randread --direct=1
--filename=/dev/vdb --time_based --runtime=10

randread: (g=0): rw=randread, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=sync, iodepth=1
fio-3.14
Starting 1 process
Jobs: 1 (f=1): [r(1)][100.0%][r=260MiB/s][r=66.7k IOPS][eta 00m:00s]
randread: (groupid=0, jobs=1): err= 0: pid=422: Tue Feb 18 11:14:47 2020
  read: IOPS=68.5k, BW=267MiB/s (280MB/s)(2674MiB/10001msec)
    clat (usec): min=10, max=966, avg=13.60, stdev= 3.49
     lat (usec): min=10, max=966, avg=13.70, stdev= 3.50
    clat percentiles (nsec):
     |  1.00th=[11200],  5.00th=[11968], 10.00th=[11968], 20.00th=[12224],
     | 30.00th=[12992], 40.00th=[13504], 50.00th=[13760], 60.00th=[13888],
     | 70.00th=[14016], 80.00th=[14144], 90.00th=[14272], 95.00th=[14656],
     | 99.00th=[20352], 99.50th=[23936], 99.90th=[35072], 99.95th=[36096],
     | 99.99th=[47872]
   bw (  KiB/s): min=265456, max=296456, per=100.00%, avg=274229.05, stdev=13048.14, samples=19
   iops        : min=66364, max=74114, avg=68557.26, stdev=3262.03, samples=19
  lat (usec)   : 20=98.84%, 50=1.15%, 100=0.01%, 250=0.01%, 500=0.01%
  lat (usec)   : 750=0.01%, 1000=0.01%
  cpu          : usr=8.24%, sys=21.15%, ctx=684669, majf=0, minf=10
  IO depths    : 1=100.0%, 2=0.0%, 4=0.0%, 8=0.0%, 16=0.0%, 32=0.0%, >=64=0.0%
     submit    : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
     complete  : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
     issued rwts: total=684611,0,0,0 short=0,0,0,0 dropped=0,0,0,0
     latency   : target=0, window=0, percentile=100.00%, depth=1

Run status group 0 (all jobs):
   READ: bw=267MiB/s (280MB/s), 267MiB/s-267MiB/s (280MB/s-280MB/s), io=2674MiB (2804MB), run=10001-10001msec

Disk stats (read/write):
  vdb: ios=677855/0, merge=0/0, ticks=7026/0, in_queue=0, util=99.04%

Signed-off-by: Sergio Lopez <slp@redhat.com>
This commit is contained in:
Sergio Lopez 2020-02-17 08:52:25 -05:00 committed by Rob Bradford
parent 0e4e27ea9d
commit 5c06b7f862
3 changed files with 71 additions and 13 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
src/bin/vhost_user_blk.rs
View File

@ -26,7 +26,7 @@ fn main() {
.help(
"vhost-user-block backend parameters \"image=<image_path>,\
sock=<socket_path>,num_queues=<number_of_queues>,\
readonly=true|false,direct=true|false\"",
readonly=true|false,direct=true|false,poll_queue=true|false\"",
)
.takes_value(true)
.min_values(1),

2
src/main.rs
View File

@ -289,7 +289,7 @@ fn create_app<'a, 'b>(
.help(
"vhost-user-block backend parameters \"image=<image_path>,\
sock=<socket_path>,num_queues=<number_of_queues>,\
readonly=true|false,direct=true|false\"",
readonly=true|false,direct=true|false,poll_queue=true|false\"",
)
.takes_value(true)
.conflicts_with_all(&["net-backend", "kernel"])

80
vhost_user_block/src/lib.rs
View File

@ -28,6 +28,7 @@ use std::path::PathBuf;
use std::process;
use std::slice;
use std::sync::{Arc, RwLock};
use std::time::Instant;
use std::vec::Vec;
use std::{convert, error, fmt, io};
use vhost_rs::vhost_user::message::*;
@ -42,6 +43,10 @@ const QUEUE_SIZE: usize = 1024;
const SECTOR_SHIFT: u8 = 9;
const SECTOR_SIZE: u64 = (0x01 as u64) << SECTOR_SHIFT;
const BLK_SIZE: u32 = 512;
// Current (2020) enterprise SSDs have a latency lower than 30us.
// Polling for 50us should be enough to cover for the device latency
// and the overhead of the emulation layer.
const POLL_QUEUE_US: u128 = 50;
trait DiskFile: Read + Seek + Write + Send + Sync {}
impl<D: Read + Seek + Write + Send + Sync> DiskFile for D {}
@ -67,6 +72,8 @@ pub enum Error {
ParseReadOnlyParam,
/// Failed parsing fs number of queues parameter.
ParseBlkNumQueuesParam(std::num::ParseIntError),
/// Failed to parse the poll_queue parameter.
ParsePollQueueParam,
/// Failed to handle event other than input event.
HandleEventNotEpollIn,
/// Failed to create kill eventfd
@ -97,12 +104,19 @@ pub struct VhostUserBlkBackend {
disk_nsectors: u64,
config: virtio_blk_config,
rdonly: bool,
poll_queue: bool,
event_idx: bool,
kill_evt: EventFd,
}
impl VhostUserBlkBackend {
pub fn new(image_path: String, num_queues: usize, rdonly: bool, direct: bool) -> Result<Self> {
pub fn new(
image_path: String,
num_queues: usize,
rdonly: bool,
direct: bool,
poll_queue: bool,
) -> Result<Self> {
let mut options = OpenOptions::new();
options.read(true);
options.write(!rdonly);
@ -139,6 +153,7 @@ impl VhostUserBlkBackend {
disk_nsectors: nsectors,
config,
rdonly,
poll_queue,
event_idx: false,
kill_evt: EventFd::new(EFD_NONBLOCK).map_err(Error::CreateKillEventFd)?,
})
@ -180,12 +195,21 @@ impl VhostUserBlkBackend {
}
}
if let Some(used_idx) = vring.mut_queue().add_used(mem, head.index, len) {
let used_event = vring.mut_queue().get_used_event(mem);
if vring.needs_notification(Wrapping(used_idx), used_event) {
debug!("signalling queue");
vring.signal_used_queue().unwrap();
if self.event_idx {
if let Some(used_idx) = vring.mut_queue().add_used(mem, head.index, len) {
let used_event = vring.mut_queue().get_used_event(mem);
if vring.needs_notification(Wrapping(used_idx), used_event) {
debug!("signalling queue");
vring.signal_used_queue().unwrap();
} else {
debug!("omitting signal (event_idx)");
}
used_any = true;
}
} else {
debug!("signalling queue");
vring.mut_queue().add_used(mem, head.index, len);
vring.signal_used_queue().unwrap();
used_any = true;
}
}
@ -248,13 +272,38 @@ impl VhostUserBackend for VhostUserBlkBackend {
match device_event {
q if device_event < self.config.num_queues => {
let mut vring = vrings[q as usize].write().unwrap();
if self.process_queue(&mut vring) && self.event_idx {
if let Some(mem) = self.mem.as_ref() {
vring.mut_queue().update_avail_event(mem);
// Check the queue again to ensure there are no pending request
self.process_queue(&mut vring);
if self.poll_queue {
// Actively poll the queue until POLL_QUEUE_US has passed
// without seeing a new request.
let mut now = Instant::now();
loop {
if self.process_queue(&mut vring) {
now = Instant::now();
} else if now.elapsed().as_micros() > POLL_QUEUE_US {
break;
}
}
}
if self.event_idx {
// vm-virtio's Queue implementation only checks avail_index
// once, so to properly support EVENT_IDX we need to keep
// calling process_queue() until it stops finding new
// requests on the queue.
loop {
vring
.mut_queue()
.update_avail_event(self.mem.as_ref().unwrap());
if !self.process_queue(&mut vring) {
break;
}
}
} else {
// Without EVENT_IDX, a single call is enough.
self.process_queue(&mut vring);
}
Ok(false)
}
_ => Err(Error::HandleEventUnknownEvent.into()),
@ -284,6 +333,7 @@ pub struct VhostUserBlkBackendConfig<'a> {
pub num_queues: usize,
pub readonly: bool,
pub direct: bool,
pub poll_queue: bool,
}
impl<'a> VhostUserBlkBackendConfig<'a> {
@ -295,6 +345,7 @@ impl<'a> VhostUserBlkBackendConfig<'a> {
let mut num_queues_str: &str = "";
let mut readonly: bool = false;
let mut direct: bool = false;
let mut poll_queue: bool = true;
for param in params_list.iter() {
if param.starts_with("image=") {
@ -313,6 +364,11 @@ impl<'a> VhostUserBlkBackendConfig<'a> {
Ok(b) => b,
Err(_) => return Err(Error::ParseDirectParam),
}
} else if param.starts_with("poll_queue=") {
poll_queue = match param[11..].parse::<bool>() {
Ok(b) => b,
Err(_) => return Err(Error::ParsePollQueueParam),
}
}
}
@ -334,6 +390,7 @@ impl<'a> VhostUserBlkBackendConfig<'a> {
num_queues,
readonly,
direct,
poll_queue,
})
}
}
@ -353,6 +410,7 @@ pub fn start_block_backend(backend_command: &str) {
backend_config.num_queues,
backend_config.readonly,
backend_config.direct,
backend_config.poll_queue,
)
.unwrap(),
));