[trying a 2nd time without attachments]
This is a follow-on to the discussion in the 12/3 performance meeting. I
wanted to share what we've done with IO path analysis and LTTng so far, I'd
like to see if we can establish a general methodology we could use to go after
different areas.
We've been characterizing memstore IO path for a while, using VTune and other
tools. The idea with this tracing was to get a high level a->b->c set of
points and timings and drill down in areas which are larger contributors to
latency. Looking from the point of view of the client (i.e. rados bench),
there's still some gaps in the latency being totaled up by the trace processing
script. Basically:
1) Instrument code, install, run benchmark w/ lttng tracing enabled
............ qd=1, single osd, memstore, logging disabled, 4K writes .......
Total time run: 20.001899
Total writes made: 16136
Write size: 4096
Bandwidth (MB/sec): 3.151
Stddev Bandwidth: 0.721435
Max bandwidth (MB/sec): 3.39453
Min bandwidth (MB/sec): 0
Average Latency: 0.00123722
Stddev Latency: 0.000103216
Max latency: 0.002637
Min latency: 0.000401
2) Grab the lttng trace:
[09:34:23.661871204] (+0.000003133) cephtestnode1 ioctximpl:aio_write: { cpu_id
= 12 }, { pthread_id = 140262776686656 }, { oid =
"benchmark_data_cephtestnode1_37038_object16" } [09:34:23.661873274]
(+0.000002070) cephtestnode1 ioctximpl:queue_aio_write: { cpu_id = 12 }, {
pthread_id = 140262776686656 }, { num = 0 } .....
[09:34:23.662166430] (+0.000206888) cephtestnode1 osd:ms_fast_dispatch: {
cpu_id = 19 }, { pthread_id = 139986434324224 }, { oid =
"benchmark_data_cephtestnode1_37038_object16", type = 8, num = 4107, tid = 17,
inc = 0 } [09:34:23.662176713] (+0.000010283) cephtestnode1
oprequest:set_rmw_flags: { cpu_id = 19 }, { pthread_id = 139986434324224 }, {
type = 8, num = 4107, tid = 17, inc = 0, flag = 0x4, old_rmw_flags = 0x0,
new_rmw_flags = 0x4 } [09:34:23.662193699] (+0.000016986) cephtestnode1
osd:enqueue_op: { cpu_id = 19 }, { pthread_id = 139986434324224 }, { oid =
"benchmark_data_cephtestnode1_37038_object16" } ......
3) Process it with a perl script, using the OID (traced in some spots) to track
a single op. When an op completes (hits the last trace point that we have
instrumented) we grab all the time stamps for the previous tracepoints and
gather the following info, which shows the thread ID, and the delta in time
(microseconds) since the last trace. Example:
Op Transition list for benchmark_data_cephtestnode1_37038_object53, 18
states
- [ ThrID us ]
- [ 86656 +0000] Start-> ioctximpl:aio_write:
- [ 86656 +0010] -> ioctximpl:queue_aio_write:
- [ 24224 +0300] -> osd:ms_fast_dispatch:
- [ 24224 +0011] -> oprequest:set_rmw_flags:
- [ 24224 +0017] -> osd:enqueue_op:
- [ 24224 +0006] -> osd:shard_index_enqueue:
- [ 24224 +0013] -> pg:queue_op:
- [ 94688 +0068] -> osd:opwq_process_start:
- [ 94688 +0005] -> osd:dequeue_op:
- [ 94688 +0003] -> mark_flagpt:reached_pg
- [ 94688 +0049] -> mark_flagpt:started
- [ 94688 +0005] -> mark_flagpt:started_2
- [ 94688 +0005] -> osd:prepare_tx_enter:
- [ 94688 +0003] -> osd:do_osd_op_pre:
- [ 94688 +0002] -> osd:do_osd_op_pre_write:
- [ 94688 +0015] -> osd:do_osd_op_post:
- [ 94688 +0026] -> osd:prepare_tx_exit:
- [ 94688 +0188] -> osd:opwq_process_finish:
=== > Total: 726 us
In order to see a trace of the entire run (vs just looking at 1 op or an
average across thousands) I generated a CSV with all of the ops as well, and
stuck them in a graph.
The total time tracked by the script is about 750us and rados bench reports an
average latency of about 1.2ms. So there's still some missing parts, I believe
a fair amount is in the commit being seen by the client. The IOPS measured
by the script (counting the 'process_finish' traces) match closely with what
rados bench is reporting.
We have a lot of stuff turned off by using single replica and memstore, but it
seems the latency tracked here will still contribute to those configurations.
Any thoughts on how to better track a single op back to the client thread would
be welcomed, as well as any other ways of doing stuff.. Thanks!
I've attached some files with more detail but I've had troubles in the past
with attachments so not sure they'll go through.
Thanks,
Stephen
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