[Mochi-devel] RPC latency question

Fri Oct 29 14:20:53 CDT 2021

Hello,

I apologize in advance for the winding nature of this email. I’m not sure how to ask my question without explaining the story of my results some.

I’m doing some characterization of our server performance under load, and I have a quirk of performance that I wanted to run by people to see if they make sense. My testing so far has been to iteratively send batches of RPCs using margo_iforward, and then measure the wait time until they are all complete. On the server side, handling the RPC includes a margo_bulk_transfer as a pull initiated on the server to pull (for now) 8 bytes. The payload of the RPC request is about 500 bytes, and the response payload is 4 bytes.

I’ve isolated my results down to one server rank and one client rank, because it’s an easier starting point to reason from. Below is a graph of some of my initial results. These results are from Frontera. The median times are good (a single RPC takes on the order of 10s of microseconds, which seems fantastic). However, the outliers are fairly high (note the log scale of the y-axis). With only one RPC per timestep, for example, there is a 30x spread between the median and the max.

[cid:5EB5DF63-97AA-48FC-8BBE-E666E933D79F]

I was hoping (expecting) the first timestep would be where the long replies resided, but that turned out not to be the case. Below are traces from the 1 RPC (blue) and 2 RPC  (orange) per timestep cases, 5 trials of 10 timesteps for each case (normalized to fix the same x-axis):

[cid:EB8719E1-6342-4D41-B8EC-791861C5B8D4]

What strikes me is how consistent these results are across trials. For the 1 RPC per timestep case, the 3rd and 7th timestep are consistently slow (and the rest are fast). For the 2 RPC per timestep case, the 2nd and 4th timestep are always slow and sometimes the 10th is. These results are repeatable with very rare variation.

For the single RPC case, I recorded some timers on the server side, and attempted to overlay them with the client side (there is some unknown offset, but probably on the order of 10s of microseconds at worst, given the pattern):

[cid:14487613-0AEF-4E0C-B5A8-61C0336A4C59]

I blew up the first few timesteps of one of the trials:
[cid:7EC4CC0B-395C-4360-BEAF-5A7EDE06618C]

The different colors are different segments of the handler, but there doesn’t seem to be anything too interesting going on inside the handler. So it looks like the time is being introduced before the 3rd RPC handler starts, based on the where the gap appears on the server side.

To try and isolate any dataspaces-specific behavior, I created a pure Margo test case that just sends a single rpc of the same size as dataspaces iteratively, whre the server side does an 8-byte bulk transfer initiated by the server, and sends a response. The results are similar, except that it is now the 4th and 8th timestep that are slow (and the first timestep is VERY long, presumably because rxm communication state is being established. DataSpaces has an earlier RPC in its init that was absorbing this latency).

I got margo profiling results for this test case:

```
3
18446744025556676964,ofi+verbs;ofi_rxm://192.168.72.245:39573
0xa2a1,term_rpc
0x27b5,put_rpc
0xd320,__shutdown__
0x27b5 ,0.000206208,10165,18446744027256353016,0,0.041241646,0.000045538,0.025733232,200,18446744073709551615,286331153,0,18446744073709551615,286331153,0
0x27b5 ,0;0.041241646,200.000000000, 0;
0xa2a1 ,0.000009298,41633,18446744027256353016,0,0.000009298,0.000009298,0.000009298,1,18446744073709551615,286331153,0,18446744073709551615,286331153,0
0xa2a1 ,0;0.000009298,1.000000000, 0;
```

So I guess my question at this point is, is there any sensible reason why the 4th and 8th RPC sent would have a long response time? I think I’ve cleared my code on the client side and server side, so it appears to be latency being introduced by Margo, LibFabric, Argobots, or the underlying OS. I do see long timesteps occasionally after this (perhaps every 20-30 timesteps) but these are not consistent.

One last detail: this does not happen on Summit. On summit, I see about 5-7x worse single-RPC performance (250-350 microseconds per RPC), but without the intermittent long timesteps.

I can provide the minimal test case if it would be helpful. I am using APEX for timing results, and the following dependencies with Spack:

argobots at 1.1  json-c at 0.15  libfabric at 1.13.1  mercury at 2.0.1  mochi-margo at 0.9.5  rdma-core at 20

Thanks,
Philip

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