[Rd] parallel PSOCK connection latency is greater on Linux?

[Simon Urbanek]

It looks like R sockets on Linux could do with TCP_NODELAY -- without (status quo):

Unit: microseconds
                   expr      min       lq     mean  median       uq      max
 clusterEvalQ(cl, iris) 1449.997 43991.99 43975.21 43997.1 44001.91 48027.83
 neval
  1000

exactly the same machine + R but with TCP_NODELAY enabled in R_SockConnect():

Unit: microseconds
                   expr     min     lq     mean  median      uq      max neval
 clusterEvalQ(cl, iris) 156.125 166.41 180.8806 170.247 174.298 5322.234  1000

Cheers,
Simon


> On 2/11/2020, at 3:39 AM, Jeff <jeff using vtkellers.com> wrote:
> 
> I'm exploring latency overhead of parallel PSOCK workers and noticed that serializing/unserializing data back to the main R session is significantly slower on Linux than it is on Windows/MacOS with similar hardware. Is there a reason for this difference and is there a way to avoid the apparent additional Linux overhead?
> 
> I attempted to isolate the behavior with a test that simply returns an existing object from the worker back to the main R session.
> 
> library(parallel)
> library(microbenchmark)
> gcinfo(TRUE)
> cl <- makeCluster(1)
> (x <- microbenchmark(clusterEvalQ(cl, iris), times = 1000, unit = "us"))
> plot(x$time, ylab = "microseconds")
> head(x$time, n = 10)
> 
> On Windows/MacOS, the test runs in 300-500 microseconds depending on hardware. A few of the 1000 runs are an order of magnitude slower but this can probably be attributed to garbage collection on the worker.
> 
> On Linux, the first 5 or so executions run at comparable speeds but all subsequent executions are two orders of magnitude slower (~40 milliseconds).
> 
> I see this behavior across various platforms and hardware combinations:
> 
> Ubuntu 18.04 (Intel Xeon Platinum 8259CL)
> Linux Mint 19.3 (AMD Ryzen 7 1800X)
> Linux Mint 20 (AMD Ryzen 7 3700X)
> Windows 10 (AMD Ryzen 7 4800H)
> MacOS 10.15.7 (Intel Core i7-8850H)
> 
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