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% File src/library/base/man/Memory-limits.Rd
% Part of the R package, http://www.R-project.org
% Copyright 1995-2007 R Core Development Team
% Distributed under GPL 2 or later

\name{Memory-limits}
\alias{Memory-limits}
\concept{memory}
\concept{limits}
\title{Memory Limits in R}
\description{
  \R holds objects it is using in memory.  This help file documents the
  current design limitations on large objects: these differ between
  32-bit and 64-bit builds of \R.
}
\details{
  \R holds all objects in memory, and there are limits based on the
  amount of memory that can be used by all objects:
  \itemize{
    \item There may be limits on the size of the heap and the number of
    cons cells allowed -- see \code{\link{Memory}} -- but these are
    usually not imposed.
    \item There is a limit on the address space of a single process such
    as the \R executable.  This is system-specific.
    \item The environment may impose limitations on the resources
    available to a single process: Windows' versions of \R do so directly.
  }
  Error messages beginning \code{cannot allocate vector of size}
  indicate a failure to obtain memory, either because the size exceeded
  the address-space limit for a process or, more likely, because the
  system was unable to provide the memory.  Note that on a 32-bit OS
  there may well be enough free memory available, but not a large enough
  contiguous block of address space into which to map it.

  There are also limits on individual objects.  On all versions of \R,
  the maximum length (number of elements) of a vector is
  \eqn{2^{31} - 1 \approx 2\thinspace 10^9}{2^31 - 1 ~ 2*10^9}, as
  lengths are stored as signed integers.  In addition, the storage space
  cannot exceed the address limit, and if you try to exceed that limit,
  the error message begins \code{cannot allocate vector of length}.
  The number of characters in a character string is in theory only
  limited by the address space.
}
\section{Unix}{
  The address-space limit is system-specific: 32-bit OSes
  imposes a limit of no more than 4Gb: it is often 3Gb or less.  Running
  32-bit executables on a 64-bit OS will have the similar limits: 64-bit
  executables will have an essentially infinite  system-specific limit.

  See the OS/shell's help on commands such as \code{limit} or
  \code{ulimit} for how to impose limitations on the resources available
  to a single process.  For example a \command{bash} user could use
\preformatted{
ulimit -t 600 -m 2000000
}
  whereas a \command{csh} user might use
\preformatted{
limit cputime 10m
limit memoryuse 2048m
}
to limit a process to 10 minutes of CPU time and (around) 2GB of memory.
}
\section{Windows}{
 The address space limit is 2Gb under 32-bit Windows unless the OS's
 default has been changed to allow more (up to 3Gb).  See
 \url{http://www.microsoft.com/whdc/system/platform/server/PAE/PAEmem.mspx}
 and
 \url{http://msdn.microsoft.com/en-us/library/bb613473\%28VS.85\%29.aspx}.
 Under most 64-bit versions of Windows the limit is 4Gb: for some older
 ones it is 2Gb.
 
 Under Windows, \R imposes limits on the total memory allocation
 available to a single session as the OS provides no way to do so: see
 \code{\link{memory.size}} and \code{\link{memory.limit}}.
}
\seealso{
  \code{\link{object.size}(a)} for the (approximate) size of \R object
  \code{a}.
}
\keyword{environment}