Hi! On 2022-11-11T15:35:44+0100, Richard Biener via Fortran wrote: > On Fri, Nov 11, 2022 at 3:13 PM Thomas Schwinge wrote: >> For example, for Fortran code like: >> >> write (*,*) "Hello world" >> >> ..., 'gfortran' creates: >> >> struct __st_parameter_dt dt_parm.0; >> >> try >> { >> dt_parm.0.common.filename = &"source-gcc/libgomp/testsuite/libgomp.oacc-fortran/print-1_.f90"[1]{lb: 1 sz: 1}; >> dt_parm.0.common.line = 29; >> dt_parm.0.common.flags = 128; >> dt_parm.0.common.unit = 6; >> _gfortran_st_write (&dt_parm.0); >> _gfortran_transfer_character_write (&dt_parm.0, &"Hello world"[1]{lb: 1 sz: 1}, 11); >> _gfortran_st_write_done (&dt_parm.0); >> } >> finally >> { >> dt_parm.0 = {CLOBBER(eol)}; >> } >> >> The issue: the stack object 'dt_parm.0' is a half-KiB in size (yes, >> really! -- there's a lot of state in Fortran I/O apparently). That's a >> problem for GPU execution -- here: OpenACC/nvptx -- where typically you >> have small stacks. (For example, GCC/OpenACC/nvptx: 1 KiB per thread; >> GCC/OpenMP/nvptx is an exception, because of its use of '-msoft-stack' >> "Use custom stacks instead of local memory for automatic storage".) >> >> Now, the Nvidia Driver tries to accomodate for such largish stack usage, >> and dynamically increases the per-thread stack as necessary (thereby >> potentially reducing parallelism) -- if it manages to understand the call >> graph. In case of libgfortran I/O, it evidently doesn't. Not being able >> to disprove existance of recursion is the common problem, as I've read. >> At run time, via 'CU_JIT_INFO_LOG_BUFFER' you then get, for example: >> >> warning : Stack size for entry function 'MAIN__$_omp_fn$0' cannot be statically determined >> >> That's still not an actual problem: if the GPU kernel's stack usage still >> fits into 1 KiB. Very often it does, but if, as happens in libgfortran >> I/O handling, there is another such 'dt_parm' put onto the stack, the >> stack then overflows; device-side SIGSEGV. >> >> (There is, by the way, some similar analysis by Tom de Vries in >> "[nvptx, openacc, openmp, testsuite] >> Recursive tests may fail due to thread stack limit".) >> >> Of course, you shouldn't really be doing I/O in GPU kernels, but people >> do like their occasional "'printf' debugging", so we ought to make that >> work (... without pessimizing any "normal" code). >> >> I assume that generally reducing the size of 'dt_parm' etc. is out of >> scope. >> >> There is a way to manually set a per-thread stack size, but it's not >> obvious which size to set: that sizes needs to work for the whole GPU >> kernel, and should be as low as possible (to maximize parallelism). >> I assume that even if GCC did an accurate call graph analysis of the GPU >> kernel's maximum stack usage, that still wouldn't help: that's before the >> PTX JIT does its own code transformations, including stack spilling. >> >> There exists a 'CU_JIT_LTO' flag to "Enable link-time optimization >> (-dlto) for device code". This might help, assuming that it manages to >> simplify the libgfortran I/O code such that the PTX JIT then understands >> the call graph. But: that's available only starting with recent >> CUDA 11.4, so not a general solution -- if it works at all, which I've >> not tested. >> >> Similarly, we could enable GCC's LTO for device code generation -- but >> that's a big project, out of scope at this time. And again, we don't >> know if that at all helps this case. >> >> I see a few options: >> >> (a) Figure out what it is in the libgfortran I/O implementation that >> causes "Stack size [...] cannot be statically determined", and re-work >> that code to avoid that, or even disable certain things for nvptx, if >> feasible. > Shrink st_parameter_dt (it's part of the ABI though, kind of). Lots of the > bloat is from things that are unused for simpler I/O cases (so some > "inheritance" could help), and lots of the bloat is from using > string/length pairs using char * + size_t for what looks like could be > encoded a lot more efficiently. > > There's probably not much low-hanging fruit. (Similarly comments in Janne's email.) Well, as had to be expected, libgfortran I/O is really just one example, but the underlying problem may also be triggered in other ways (via other newlib/libc functions, for example). So, really a generic solution seems to be called for. >> (b) Also for GCC/OpenACC/nvptx use the GCC/OpenMP/nvptx '-msoft-stack'. >> I don't really want to do that however: it does introduce a bit of >> complexity in all the generated device code and run-time overhead that we >> generally would like to avoid. Directly using '-msoft-stack' isn't actually possible: it does implement "one stack per 32-threads warp", but for OpenACC we need "one stack per thread of a warp" (that is, each OpenACC 'vector' independently), and pre-allocating from device memory all those stacks (which may be a lot!) I foresee to really negatively impact overall performance? >> (c) I'm contemplating a tweak/compiler pass for transforming such large >> stack objects into heap allocation (during nvptx offloading compilation). >> 'malloc'/'free' do exist; they're slow, but that's not a problem for the >> code paths this is to affect. (Might also add some compile-time >> diagnostic, of course.) Could maybe even limit this to only be used >> during libgfortran compilation? This is then conceptually a bit similar >> to (b), but localized to relevant parts only. Has such a thing been done >> before in GCC, that I could build upon? >> >> Any other clever ideas? > Converting to heap allocation is difficult outside of the frontend and you > have to be very careful with memleaks. Heh, in fact it seems to be pretty simple! (Famous last words?) See "[WIP] nvptx: '-mframe-malloc-threshold', '-Wframe-malloc-threshold'" attached. What do people think about such a thing? Still to be discussed are '-Wframe-malloc-threshold' (default-on vs. '-Wextra'; or '-fopt-info' 'missed: [...]' or 'note: [...]' instead?), default value for '-mframe-malloc-threshold=[...]' (potentially different for GCC/nvptx target libraries build vs. user-compiled code?), etc. > The library is written in C and > I see heap allocated temporaries there but in at least one > place a stack one is used: > > void > st_endfile (st_parameter_filepos *fpp) > { > ... > if (u->current_record) > { > st_parameter_dt dtp; > dtp.common = fpp->common; > memset (&dtp.u.p, 0, sizeof (dtp.u.p)); > dtp.u.p.current_unit = u; > next_record (&dtp, 1); > > that might be a mistake though - maybe it's enough to change that > to a heap allocation? It might be also totally superfluous since > only 'u' should matter here ... (not sure if the above is the case > you are running into). (Have not yet looked into that; won't solve the general issue.) Grüße Thomas ----------------- Siemens Electronic Design Automation GmbH; Anschrift: Arnulfstraße 201, 80634 München; Gesellschaft mit beschränkter Haftung; Geschäftsführer: Thomas Heurung, Frank Thürauf; Sitz der Gesellschaft: München; Registergericht München, HRB 106955