parallel PSOCK connection latency is greater on Linux?

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 at 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)