diff --git a/R/test.data.table.R b/R/test.data.table.R
index 0d19a7cf1..1c212f58b 100644
--- a/R/test.data.table.R
+++ b/R/test.data.table.R
@@ -1,6 +1,8 @@
 test.data.table = function(script="tests.Rraw", verbose=FALSE, pkg=".", silent=FALSE, showProgress=interactive()&&!silent,
-                           memtest=as.logical(Sys.getenv("TEST_DATA_TABLE_MEMTEST", "FALSE"))) {
+                           memtest=Sys.getenv("TEST_DATA_TABLE_MEMTEST", 0)) {
   stopifnot(isTRUEorFALSE(verbose), isTRUEorFALSE(silent), isTRUEorFALSE(showProgress))
+  memtest = as.integer(memtest)
+  stopifnot(length(memtest)==1L, memtest %in% 0:2)
   if (exists("test.data.table", .GlobalEnv, inherits=FALSE)) {
     # package developer
     # nocov start
@@ -124,7 +126,7 @@ test.data.table = function(script="tests.Rraw", verbose=FALSE, pkg=".", silent=F
   on.exit(setwd(owd))
   
   if (memtest) {
-    catf("\n***\n*** memtest=TRUE. This should be the first task in a fresh R session for best results. Ctrl-C now if not.\n***\n\n")
+    catf("\n***\n*** memtest=%d. This should be the first task in a fresh R session for best results. Ctrl-C now if not.\n***\n\n", memtest)
     if (is.na(ps_mem())) stopf("memtest intended for Linux. Step through ps_mem() to see what went wrong.")
   }
 
@@ -277,8 +279,9 @@ test = function(num,x,y=TRUE,error=NULL,warning=NULL,message=NULL,output=NULL,no
        took = proc.time()[3L]-lasttime  # so that prep time between tests is attributed to the following test
        timings[as.integer(num), `:=`(time=time+took, nTest=nTest+1L), verbose=FALSE]
        if (memtest) {
-         gc() # force gc so we can find tests that use relatively larger amounts of RAM
+         if (memtest==1L) gc()  # see #5515 for before/after
          timings[as.integer(num), RSS:=max(ps_mem(),RSS), verbose=FALSE]
+         if (memtest==2L) gc()
        }
        assign("lasttime", proc.time()[3L], parent.frame(), inherits=TRUE)  # after gc() to exclude gc() time from next test when memtest
     } )
@@ -293,7 +296,7 @@ test = function(num,x,y=TRUE,error=NULL,warning=NULL,message=NULL,output=NULL,no
     # not be flushed to the output upon segfault, depending on OS).
   } else {
     # not `test.data.table` but developer running tests manually; i.e. `cc(F); test(...)`
-    memtest = FALSE          # nocov
+    memtest = 0L             # nocov
     filename = NA_character_ # nocov
     foreign = FALSE          # nocov ; assumes users of 'cc(F); test(...)' has LANGUAGE=en
     showProgress = FALSE     # nocov
diff --git a/man/test.data.table.Rd b/man/test.data.table.Rd
index 90f58593a..2df2a3284 100644
--- a/man/test.data.table.Rd
+++ b/man/test.data.table.Rd
@@ -8,7 +8,7 @@
 test.data.table(script = "tests.Rraw", verbose = FALSE, pkg = ".",
                 silent = FALSE,
                 showProgress = interactive() && !silent,
-                memtest = as.logical(Sys.getenv("TEST_DATA_TABLE_MEMTEST", "FALSE")))
+                memtest = Sys.getenv("TEST_DATA_TABLE_MEMTEST", 0))
 }
 \arguments{
 \item{script}{ Run arbitrary R test script. }
@@ -16,7 +16,7 @@ test.data.table(script = "tests.Rraw", verbose = FALSE, pkg = ".",
 \item{pkg}{ Root directory name under which all package content (ex: DESCRIPTION, src/, R/, inst/ etc..) resides. Used only in \emph{dev-mode}. }
 \item{silent}{ Controls what happens if a test fails. Like \code{silent} in \code{\link{try}}, \code{TRUE} causes the error message to be suppressed and \code{FALSE} to be returned, otherwise the error is returned. }
 \item{showProgress}{ Output 'Running test <n> ...\\r' at the start of each test? }
-\item{memtest}{ Measure and report memory usage of tests rather than time taken. Intended for and tested on Linux. }
+\item{memtest}{ Measure and report memory usage of tests (1:gc before ps, 2:gc after ps) rather than time taken (0) by default. Intended for and tested on Linux. See PR #5515 for more details. }
 }
 \details{
   Runs a series of tests. These can be used to see features and examples of usage, too. Running test.data.table will tell you the full location of the test file(s) to open.