| Buffer mapping patterns |
| ----------------------- |
| |
| There are two main strategies the driver has for CPU access to GL buffer |
| objects. One is that the GL calls allocate temporary storage and blit to the GPU |
| at |
| ``glBufferSubData()``/``glBufferData()``/``glFlushMappedBufferRange()``/``glUnmapBuffer()`` |
| time. This makes the behavior easily match. However, this may be more costly |
| than direct mapping of the GL BO on some platforms, and is essentially not |
| available to tiling GPUs (since tiling involves running through the command |
| stream multiple times). Thus, GL has additional interfaces to help make it so |
| apps can directly access memory while avoiding implicit blocking on the GPU |
| rendering from those BOs. |
| |
| Rendering engines have a variety of knobs to set on those GL interfaces for data |
| upload, and as a whole they seem to take just about every path available. Let's |
| look at some examples to see how they might constrain GL driver buffer upload |
| behavior. |
| |
| Portal 2 |
| ======== |
| |
| .. code-block:: console |
| |
| 1030842 glXSwapBuffers(dpy = 0x82a8000, drawable = 20971540) |
| 1030876 glBufferDataARB(target = GL_ELEMENT_ARRAY_BUFFER, size = 65536, data = NULL, usage = GL_DYNAMIC_DRAW) |
| 1030877 glBufferSubData(target = GL_ELEMENT_ARRAY_BUFFER, offset = 0, size = 576, data = blob(576)) |
| 1030896 glDrawRangeElementsBaseVertex(mode = GL_TRIANGLES, start = 0, end = 526, count = 252, type = GL_UNSIGNED_SHORT, indices = NULL, basevertex = 0) |
| 1030915 glDrawRangeElementsBaseVertex(mode = GL_TRIANGLES, start = 0, end = 19657, count = 36, type = GL_UNSIGNED_SHORT, indices = 0x1f8, basevertex = 0) |
| 1030917 glBufferDataARB(target = GL_ARRAY_BUFFER, size = 1572864, data = NULL, usage = GL_DYNAMIC_DRAW) |
| 1030918 glBufferSubData(target = GL_ARRAY_BUFFER, offset = 0, size = 128, data = blob(128)) |
| 1030919 glBufferSubData(target = GL_ELEMENT_ARRAY_BUFFER, offset = 576, size = 12, data = blob(12)) |
| 1030936 glDrawRangeElementsBaseVertex(mode = GL_TRIANGLES, start = 0, end = 3, count = 6, type = GL_UNSIGNED_SHORT, indices = 0x240, basevertex = 0) |
| 1030937 glBufferSubData(target = GL_ARRAY_BUFFER, offset = 128, size = 128, data = blob(128)) |
| 1030938 glBufferSubData(target = GL_ELEMENT_ARRAY_BUFFER, offset = 588, size = 12, data = blob(12)) |
| 1030940 glDrawRangeElementsBaseVertex(mode = GL_TRIANGLES, start = 4, end = 7, count = 6, type = GL_UNSIGNED_SHORT, indices = 0x24c, basevertex = 0) |
| [... repeated draws at increasing offsets] |
| 1033097 glXSwapBuffers(dpy = 0x82a8000, drawable = 20971540) |
| |
| From this sequence, we can see that it is important that the driver either |
| implement ``glBufferSubData()`` as a blit from a streaming uploader in sequence with |
| the ``glDraw*()`` calls (a common behavior for non-tiled GPUs, particularly those with |
| dedicated memory), or that you: |
| |
| 1) Track the valid range of the buffer so that you don't have to flush the draws |
| and synchronize on each following ``glBufferSubData()``. |
| |
| 2) Reallocate the buffer storage on ``glBufferData`` so that your first |
| ``glBufferSubData()`` of the frame doesn't stall on the last frame's |
| rendering completing. |
| |
| You can't just empty your valid range on ``glBufferData()`` unless you know that |
| the GPU access from the previous frame has completed. This pattern of |
| incrementing ``glBufferSubData()`` offsets interleaved with draws from that data |
| is common among newer Valve games. |
| |
| .. code-block:: console |
| |
| [ during setup ] |
| |
| 679259 glGenBuffersARB(n = 1, buffers = &1314) |
| 679260 glBindBufferARB(target = GL_ELEMENT_ARRAY_BUFFER, buffer = 1314) |
| 679261 glBufferDataARB(target = GL_ELEMENT_ARRAY_BUFFER, size = 3072, data = NULL, usage = GL_STATIC_DRAW) |
| 679264 glMapBufferRange(target = GL_ELEMENT_ARRAY_BUFFER, offset = 0, length = 3072, access = GL_MAP_WRITE_BIT | GL_MAP_FLUSH_EXPLICIT_BIT) = 0xd7384000 |
| 679269 glFlushMappedBufferRange(target = GL_ELEMENT_ARRAY_BUFFER, offset = 0, length = 3072) |
| 679270 glUnmapBuffer(target = GL_ELEMENT_ARRAY_BUFFER) = GL_TRUE |
| |
| [... setup of other buffers on this binding point] |
| |
| 679343 glBindBufferARB(target = GL_ELEMENT_ARRAY_BUFFER, buffer = 1314) |
| 679344 glMapBufferRange(target = GL_ELEMENT_ARRAY_BUFFER, offset = 0, length = 768, access = GL_MAP_WRITE_BIT | GL_MAP_FLUSH_EXPLICIT_BIT) = 0xd7384000 |
| 679346 glFlushMappedBufferRange(target = GL_ELEMENT_ARRAY_BUFFER, offset = 0, length = 768) |
| 679347 glUnmapBuffer(target = GL_ELEMENT_ARRAY_BUFFER) = GL_TRUE |
| 679348 glMapBufferRange(target = GL_ELEMENT_ARRAY_BUFFER, offset = 768, length = 768, access = GL_MAP_WRITE_BIT | GL_MAP_FLUSH_EXPLICIT_BIT) = 0xd7384300 |
| 679350 glFlushMappedBufferRange(target = GL_ELEMENT_ARRAY_BUFFER, offset = 0, length = 768) |
| 679351 glUnmapBuffer(target = GL_ELEMENT_ARRAY_BUFFER) = GL_TRUE |
| 679352 glMapBufferRange(target = GL_ELEMENT_ARRAY_BUFFER, offset = 1536, length = 768, access = GL_MAP_WRITE_BIT | GL_MAP_FLUSH_EXPLICIT_BIT) = 0xd7384600 |
| 679354 glFlushMappedBufferRange(target = GL_ELEMENT_ARRAY_BUFFER, offset = 0, length = 768) |
| 679355 glUnmapBuffer(target = GL_ELEMENT_ARRAY_BUFFER) = GL_TRUE |
| 679356 glMapBufferRange(target = GL_ELEMENT_ARRAY_BUFFER, offset = 2304, length = 768, access = GL_MAP_WRITE_BIT | GL_MAP_FLUSH_EXPLICIT_BIT) = 0xd7384900 |
| 679358 glFlushMappedBufferRange(target = GL_ELEMENT_ARRAY_BUFFER, offset = 0, length = 768) |
| 679359 glUnmapBuffer(target = GL_ELEMENT_ARRAY_BUFFER) = GL_TRUE |
| |
| [... setup completes and we start drawing later] |
| |
| 761845 glBindBufferARB(target = GL_ELEMENT_ARRAY_BUFFER, buffer = 1314) |
| 761846 glDrawRangeElementsBaseVertex(mode = GL_TRIANGLES, start = 0, end = 323, count = 384, type = GL_UNSIGNED_SHORT, indices = NULL, basevertex = 0) |
| |
| This suggests that, for non-blitting drivers, resetting your "might be used on |
| the GPU" range after a stall could save you a bunch of additional GPU stalls |
| during setup. |
| |
| Terraria |
| ======== |
| |
| .. code-block:: console |
| |
| 167581 glXSwapBuffers(dpy = 0x3004630, drawable = 25165844) |
| |
| 167585 glBufferData(target = GL_ARRAY_BUFFER, size = 196608, data = NULL, usage = GL_STREAM_DRAW) |
| 167586 glBufferSubData(target = GL_ARRAY_BUFFER, offset = 0, size = 1728, data = blob(1728)) |
| 167588 glDrawRangeElementsBaseVertex(mode = GL_TRIANGLES, start = 0, end = 71, count = 108, type = GL_UNSIGNED_SHORT, indices = NULL, basevertex = 0) |
| 167589 glBufferData(target = GL_ARRAY_BUFFER, size = 196608, data = NULL, usage = GL_STREAM_DRAW) |
| 167590 glBufferSubData(target = GL_ARRAY_BUFFER, offset = 0, size = 27456, data = blob(27456)) |
| 167592 glDrawRangeElementsBaseVertex(mode = GL_TRIANGLES, start = 0, end = 7, count = 12, type = GL_UNSIGNED_SHORT, indices = NULL, basevertex = 0) |
| 167594 glDrawRangeElementsBaseVertex(mode = GL_TRIANGLES, start = 0, end = 3, count = 6, type = GL_UNSIGNED_SHORT, indices = NULL, basevertex = 8) |
| 167596 glDrawRangeElementsBaseVertex(mode = GL_TRIANGLES, start = 0, end = 3, count = 6, type = GL_UNSIGNED_SHORT, indices = NULL, basevertex = 12) |
| [...] |
| |
| In this game, we can see ``glBufferData()`` being used on the same array buffer |
| throughout, to get new storage so that the ``glBufferSubData()`` doesn't cause |
| synchronization. |
| |
| Don't Starve |
| ============ |
| |
| .. code-block:: console |
| |
| 7251917 glGenBuffers(n = 1, buffers = &115052) |
| 7251918 glBindBuffer(target = GL_ARRAY_BUFFER, buffer = 115052) |
| 7251919 glBufferData(target = GL_ARRAY_BUFFER, size = 144, data = blob(144), usage = GL_STREAM_DRAW) |
| 7251921 glBindBuffer(target = GL_ARRAY_BUFFER, buffer = 115052) |
| 7251928 glDrawArrays(mode = GL_TRIANGLES, first = 0, count = 6) |
| 7251930 glBindBuffer(target = GL_ARRAY_BUFFER, buffer = 114872) |
| 7251936 glDrawArrays(mode = GL_TRIANGLES, first = 0, count = 18) |
| 7251938 glGenBuffers(n = 1, buffers = &115053) |
| 7251939 glBindBuffer(target = GL_ARRAY_BUFFER, buffer = 115053) |
| 7251940 glBufferData(target = GL_ARRAY_BUFFER, size = 144, data = blob(144), usage = GL_STREAM_DRAW) |
| 7251942 glBindBuffer(target = GL_ARRAY_BUFFER, buffer = 115053) |
| 7251949 glDrawArrays(mode = GL_TRIANGLES, first = 0, count = 6) |
| 7251973 glXSwapBuffers(dpy = 0x86dd860, drawable = 20971540) |
| [... drawing next frame] |
| 7252388 glDeleteBuffers(n = 1, buffers = &115052) |
| 7252389 glDeleteBuffers(n = 1, buffers = &115053) |
| 7252390 glXSwapBuffers(dpy = 0x86dd860, drawable = 20971540) |
| |
| In this game we have a lot of tiny ``glBufferData()`` calls, suggesting that we |
| could see working set wins and possibly CPU overhead reduction by packing small |
| GL buffers in the same BO. Interestingly, the deletes of the temporary buffers |
| always happen at the end of the next frame. |
| |
| Euro Truck Simulator |
| ==================== |
| |
| .. code-block:: console |
| |
| [usage of VBO 14,15] |
| [...] |
| 885199 glXSwapBuffers(dpy = 0x379a3e0, drawable = 20971527) |
| 885203 glInvalidateBufferData(buffer = 14) |
| 885204 glInvalidateBufferData(buffer = 15) |
| [...] |
| 889330 glXSwapBuffers(dpy = 0x379a3e0, drawable = 20971527) |
| 889334 glInvalidateBufferData(buffer = 12) |
| 889335 glInvalidateBufferData(buffer = 16) |
| [...] |
| 893461 glXSwapBuffers(dpy = 0x379a3e0, drawable = 20971527) |
| 893462 glClientWaitSync(sync = 0x77eee10, flags = 0x0, timeout = 0) = GL_ALREADY_SIGNALED |
| 893463 glDeleteSync(sync = 0x780a630) |
| 893464 glFenceSync(condition = GL_SYNC_GPU_COMMANDS_COMPLETE, flags = 0) = 0x78ec730 |
| 893465 glInvalidateBufferData(buffer = 13) |
| 893466 glInvalidateBufferData(buffer = 17) |
| 893505 glBindBuffer(target = GL_COPY_READ_BUFFER, buffer = 14) |
| 893506 glMapBufferRange(target = GL_COPY_READ_BUFFER, offset = 0, length = 788, access = GL_MAP_WRITE_BIT | GL_MAP_INVALIDATE_BUFFER_BIT | GL_MAP_UNSYNCHRONIZED_BIT) = 0x7b034efd1000 |
| 893508 glUnmapBuffer(target = GL_COPY_READ_BUFFER) = GL_TRUE |
| 893509 glBindBuffer(target = GL_COPY_READ_BUFFER, buffer = 15) |
| 893510 glMapBufferRange(target = GL_COPY_READ_BUFFER, offset = 0, length = 32, access = GL_MAP_WRITE_BIT | GL_MAP_INVALIDATE_BUFFER_BIT | GL_MAP_UNSYNCHRONIZED_BIT) = 0x7b034e5df000 |
| 893512 glUnmapBuffer(target = GL_COPY_READ_BUFFER) = GL_TRUE |
| 893532 glBindVertexBuffers(first = 0, count = 2, buffers = {10, 15}, offsets = {0, 0}, strides = {52, 16}) |
| 893552 glDrawElementsInstancedBaseVertex(mode = GL_TRIANGLES, count = 18, type = GL_UNSIGNED_SHORT, indices = 0x13f280, instancecount = 1, basevertex = 25131) |
| 893609 glDrawArrays(mode = GL_TRIANGLES, first = 0, count = 6) |
| 893732 glBindVertexBuffers(first = 0, count = 1, buffers = &14, offsets = &0, strides = &48) |
| 893733 glBindBuffer(target = GL_ELEMENT_ARRAY_BUFFER, buffer = 14) |
| 893744 glDrawElementsBaseVertex(mode = GL_TRIANGLES, count = 6, type = GL_UNSIGNED_SHORT, indices = 0xf0, basevertex = 0) |
| 893759 glDrawElementsBaseVertex(mode = GL_TRIANGLES, count = 24, type = GL_UNSIGNED_SHORT, indices = 0x2e0, basevertex = 6) |
| 893786 glDrawElementsBaseVertex(mode = GL_TRIANGLES, count = 600, type = GL_UNSIGNED_SHORT, indices = 0xe87b0, basevertex = 21515) |
| 893822 glDrawArrays(mode = GL_TRIANGLES, first = 0, count = 6) |
| 893845 glBindBuffer(target = GL_COPY_READ_BUFFER, buffer = 14) |
| 893846 glMapBufferRange(target = GL_COPY_READ_BUFFER, offset = 788, length = 788, access = GL_MAP_WRITE_BIT | GL_MAP_INVALIDATE_RANGE_BIT | GL_MAP_UNSYNCHRONIZED_BIT) = 0x7b034efd1314 |
| 893848 glUnmapBuffer(target = GL_COPY_READ_BUFFER) = GL_TRUE |
| 893886 glDrawElementsInstancedBaseVertex(mode = GL_TRIANGLES, count = 18, type = GL_UNSIGNED_SHORT, indices = 0x13f280, instancecount = 1, basevertex = 25131) |
| 893943 glDrawArrays(mode = GL_TRIANGLES, first = 0, count = 6) |
| |
| At the start of this frame, buffer 14 and 15 haven't been used in the previous 2 |
| frames, and the ``GL_ARB_sync`` fence has ensured that the GPU has at least started |
| frame n-1 as the CPU starts the current frame. The first map is ``offset = 0, |
| INVALIDATE_BUFFER | UNSYNCHRONIZED``, which suggests that the driver should |
| reallocate storage for the mapping even in the ``UNSYNCHRONIZED`` case, except |
| that the buffer is definitely going to be idle, making reallocation unnecessary |
| (you may need to empty your valid range, though, to prevent unnecessary batch |
| flushes). |
| |
| Also note the use of a totally unrelated binding point for the mapping of the |
| vertex array -- you can't effectively use it as a hint for any buffer placement |
| in memory. The game does also use ``glCopyBufferSubData()``, but only on a |
| different buffer. |
| |
| |
| Plague Inc |
| ========== |
| |
| .. code-block:: console |
| |
| 1640732 glXSwapBuffers(dpy = 0xb218f20, drawable = 23068674) |
| 1640733 glClientWaitSync(sync = 0xb4141430, flags = 0x0, timeout = 0) = GL_ALREADY_SIGNALED |
| 1640734 glDeleteSync(sync = 0xb4141430) |
| 1640735 glFenceSync(condition = GL_SYNC_GPU_COMMANDS_COMPLETE, flags = 0) = 0xb4141430 |
| |
| 1640780 glBindBuffer(target = GL_ARRAY_BUFFER, buffer = 78) |
| 1640787 glBindBuffer(target = GL_ELEMENT_ARRAY_BUFFER, buffer = 79) |
| 1640788 glDrawElements(mode = GL_TRIANGLES, count = 9636, type = GL_UNSIGNED_SHORT, indices = NULL) |
| 1640795 glDrawElements(mode = GL_TRIANGLES, count = 9636, type = GL_UNSIGNED_SHORT, indices = NULL) |
| 1640813 glBindBuffer(target = GL_COPY_WRITE_BUFFER, buffer = 1096) |
| 1640814 glMapBufferRange(target = GL_COPY_WRITE_BUFFER, offset = 0, length = 67584, access = GL_MAP_WRITE_BIT | GL_MAP_FLUSH_EXPLICIT_BIT | GL_MAP_UNSYNCHRONIZED_BIT) = 0xbfef4000 |
| 1640815 glBindBuffer(target = GL_COPY_WRITE_BUFFER, buffer = 1091) |
| 1640816 glMapBufferRange(target = GL_COPY_WRITE_BUFFER, offset = 0, length = 12, access = GL_MAP_WRITE_BIT | GL_MAP_FLUSH_EXPLICIT_BIT | GL_MAP_UNSYNCHRONIZED_BIT) = 0xc3998000 |
| 1640817 glBindBuffer(target = GL_COPY_WRITE_BUFFER, buffer = 1096) |
| 1640819 glFlushMappedBufferRange(target = GL_COPY_WRITE_BUFFER, offset = 0, length = 352) |
| 1640820 glUnmapBuffer(target = GL_COPY_WRITE_BUFFER) = GL_TRUE |
| 1640821 glBindBuffer(target = GL_COPY_WRITE_BUFFER, buffer = 1091) |
| 1640823 glFlushMappedBufferRange(target = GL_COPY_WRITE_BUFFER, offset = 0, length = 12) |
| 1640824 glUnmapBuffer(target = GL_COPY_WRITE_BUFFER) = GL_TRUE |
| 1640825 glBindBuffer(target = GL_ARRAY_BUFFER, buffer = 1096) |
| 1640831 glBindBuffer(target = GL_ELEMENT_ARRAY_BUFFER, buffer = 1091) |
| 1640832 glDrawElements(mode = GL_TRIANGLES, count = 6, type = GL_UNSIGNED_SHORT, indices = NULL) |
| |
| 1640847 glBindBuffer(target = GL_COPY_WRITE_BUFFER, buffer = 1096) |
| 1640848 glMapBufferRange(target = GL_COPY_WRITE_BUFFER, offset = 352, length = 67584, access = GL_MAP_WRITE_BIT | GL_MAP_FLUSH_EXPLICIT_BIT | GL_MAP_UNSYNCHRONIZED_BIT) = 0xbfef4160 |
| 1640849 glBindBuffer(target = GL_COPY_WRITE_BUFFER, buffer = 1091) |
| 1640850 glMapBufferRange(target = GL_COPY_WRITE_BUFFER, offset = 88, length = 12, access = GL_MAP_WRITE_BIT | GL_MAP_FLUSH_EXPLICIT_BIT | GL_MAP_UNSYNCHRONIZED_BIT) = 0xc3998058 |
| 1640851 glBindBuffer(target = GL_COPY_WRITE_BUFFER, buffer = 1096) |
| 1640853 glFlushMappedBufferRange(target = GL_COPY_WRITE_BUFFER, offset = 0, length = 352) |
| 1640854 glUnmapBuffer(target = GL_COPY_WRITE_BUFFER) = GL_TRUE |
| 1640855 glBindBuffer(target = GL_COPY_WRITE_BUFFER, buffer = 1091) |
| 1640857 glFlushMappedBufferRange(target = GL_COPY_WRITE_BUFFER, offset = 0, length = 12) |
| 1640858 glUnmapBuffer(target = GL_COPY_WRITE_BUFFER) = GL_TRUE |
| 1640863 glDrawElementsBaseVertex(mode = GL_TRIANGLES, count = 6, type = GL_UNSIGNED_SHORT, indices = 0x58, basevertex = 4) |
| |
| At the start of this frame, the VBOs haven't been used in about 6 frames, and |
| the ``GL_ARB_sync`` fence has ensured that the GPU has started frame n-1. |
| |
| Note the use of ``glFlushMappedBufferRange()`` on a small fraction of the size |
| of the VBO -- it is important that a blitting driver make use of the flush |
| ranges when in explicit mode. |
| |
| Darkest Dungeon |
| =============== |
| |
| .. code-block:: console |
| |
| 938384 glXSwapBuffers(dpy = 0x377fcd0, drawable = 23068692) |
| |
| 938385 glBindBuffer(target = GL_ARRAY_BUFFER, buffer = 2) |
| 938386 glBufferData(target = GL_ARRAY_BUFFER, size = 1048576, data = NULL, usage = GL_STREAM_DRAW) |
| 938511 glBindBuffer(target = GL_ARRAY_BUFFER, buffer = 2) |
| 938512 glMapBufferRange(target = GL_ARRAY_BUFFER, offset = 0, length = 1048576, access = GL_MAP_WRITE_BIT | GL_MAP_FLUSH_EXPLICIT_BIT | GL_MAP_UNSYNCHRONIZED_BIT) = 0x7a73fcaa7000 |
| 938514 glFlushMappedBufferRange(target = GL_ARRAY_BUFFER, offset = 0, length = 512) |
| 938515 glUnmapBuffer(target = GL_ARRAY_BUFFER) = GL_TRUE |
| 938523 glBindBuffer(target = GL_ELEMENT_ARRAY_BUFFER, buffer = 1) |
| 938524 glBindBuffer(target = GL_ARRAY_BUFFER, buffer = 2) |
| 938525 glDrawElements(mode = GL_TRIANGLES, count = 24, type = GL_UNSIGNED_SHORT, indices = NULL) |
| 938527 glBindBuffer(target = GL_ARRAY_BUFFER, buffer = 2) |
| 938528 glMapBufferRange(target = GL_ARRAY_BUFFER, offset = 0, length = 1048576, access = GL_MAP_WRITE_BIT | GL_MAP_FLUSH_EXPLICIT_BIT | GL_MAP_UNSYNCHRONIZED_BIT) = 0x7a73fcaa7000 |
| 938530 glFlushMappedBufferRange(target = GL_ARRAY_BUFFER, offset = 512, length = 512) |
| 938531 glUnmapBuffer(target = GL_ARRAY_BUFFER) = GL_TRUE |
| 938539 glBindBuffer(target = GL_ELEMENT_ARRAY_BUFFER, buffer = 1) |
| 938540 glBindBuffer(target = GL_ARRAY_BUFFER, buffer = 2) |
| 938541 glDrawElements(mode = GL_TRIANGLES, count = 24, type = GL_UNSIGNED_SHORT, indices = 0x30) |
| [... more maps and draws at increasing offsets] |
| |
| Interesting note for this game, after the initial ``glBufferData()`` in the |
| frame to reallocate the storage, it unsync maps the whole buffer each time, and |
| just changes which region it flushes. The same GL buffer name is used in every |
| frame. |
| |
| Tabletop Simulator |
| ================== |
| |
| .. code-block:: console |
| |
| 1287594 glXSwapBuffers(dpy = 0x3e10810, drawable = 23068692) |
| 1287595 glClientWaitSync(sync = 0x7abf554e37b0, flags = 0x0, timeout = 0) = GL_ALREADY_SIGNALED |
| 1287596 glDeleteSync(sync = 0x7abf554e37b0) |
| 1287597 glFenceSync(condition = GL_SYNC_GPU_COMMANDS_COMPLETE, flags = 0) = 0x7abf56647490 |
| |
| 1287614 glBindBuffer(target = GL_COPY_WRITE_BUFFER, buffer = 480) |
| 1287615 glMapBufferRange(target = GL_COPY_WRITE_BUFFER, offset = 0, length = 384, access = GL_MAP_WRITE_BIT | GL_MAP_INVALIDATE_RANGE_BIT | GL_MAP_FLUSH_EXPLICIT_BIT | GL_MAP_UNSYNCHRONIZED_BIT) = 0x7abf2e79a000 |
| 1287642 glBindBuffer(target = GL_ARRAY_BUFFER, buffer = 614) |
| 1287650 glBindBuffer(target = GL_COPY_WRITE_BUFFER, buffer = 5) |
| 1287651 glBufferSubData(target = GL_COPY_WRITE_BUFFER, offset = 0, size = 1088, data = blob(1088)) |
| 1287652 glBindBuffer(target = GL_ELEMENT_ARRAY_BUFFER, buffer = 615) |
| 1287653 glDrawElements(mode = GL_TRIANGLES, count = 1788, type = GL_UNSIGNED_SHORT, indices = NULL) |
| [... more draw calls] |
| 1289055 glBindBuffer(target = GL_COPY_WRITE_BUFFER, buffer = 480) |
| 1289057 glFlushMappedBufferRange(target = GL_COPY_WRITE_BUFFER, offset = 0, length = 384) |
| 1289058 glUnmapBuffer(target = GL_COPY_WRITE_BUFFER) = GL_TRUE |
| 1289059 glBindBuffer(target = GL_ARRAY_BUFFER, buffer = 480) |
| 1289066 glDrawArrays(mode = GL_TRIANGLE_STRIP, first = 12, count = 4) |
| 1289068 glDrawArrays(mode = GL_TRIANGLE_STRIP, first = 8, count = 4) |
| 1289553 glXSwapBuffers(dpy = 0x3e10810, drawable = 23068692) |
| |
| In this app, buffer 480 gets used like this every other frame. The ``GL_ARB_sync`` |
| fence ensures that frame n-1 has started on the GPU before CPU work starts on |
| the current frame, so the unsynchronized access to the buffers is safe. |
| |
| Hollow Knight |
| ============= |
| |
| .. code-block:: console |
| |
| 1873034 glXSwapBuffers(dpy = 0x28609d0, drawable = 23068692) |
| 1873035 glClientWaitSync(sync = 0x7b1a5ca6e130, flags = 0x0, timeout = 0) = GL_ALREADY_SIGNALED |
| 1873036 glDeleteSync(sync = 0x7b1a5ca6e130) |
| 1873037 glFenceSync(condition = GL_SYNC_GPU_COMMANDS_COMPLETE, flags = 0) = 0x7b1a5ca6e130 |
| 1873038 glBindBuffer(target = GL_COPY_WRITE_BUFFER, buffer = 29) |
| 1873039 glMapBufferRange(target = GL_COPY_WRITE_BUFFER, offset = 0, length = 8640, access = GL_MAP_WRITE_BIT | GL_MAP_FLUSH_EXPLICIT_BIT | GL_MAP_UNSYNCHRONIZED_BIT) = 0x7b1a04c7e000 |
| 1873040 glBindBuffer(target = GL_COPY_WRITE_BUFFER, buffer = 30) |
| 1873041 glMapBufferRange(target = GL_COPY_WRITE_BUFFER, offset = 0, length = 720, access = GL_MAP_WRITE_BIT | GL_MAP_FLUSH_EXPLICIT_BIT | GL_MAP_UNSYNCHRONIZED_BIT) = 0x7b1a07430000 |
| 1873065 glBindBuffer(target = GL_COPY_WRITE_BUFFER, buffer = 29) |
| 1873067 glFlushMappedBufferRange(target = GL_COPY_WRITE_BUFFER, offset = 0, length = 8640) |
| 1873068 glUnmapBuffer(target = GL_COPY_WRITE_BUFFER) = GL_TRUE |
| 1873069 glBindBuffer(target = GL_COPY_WRITE_BUFFER, buffer = 30) |
| 1873071 glFlushMappedBufferRange(target = GL_COPY_WRITE_BUFFER, offset = 0, length = 720) |
| 1873072 glUnmapBuffer(target = GL_COPY_WRITE_BUFFER) = GL_TRUE |
| 1873073 glBindBuffer(target = GL_COPY_WRITE_BUFFER, buffer = 29) |
| 1873074 glMapBufferRange(target = GL_COPY_WRITE_BUFFER, offset = 8640, length = 576, access = GL_MAP_WRITE_BIT | GL_MAP_FLUSH_EXPLICIT_BIT | GL_MAP_UNSYNCHRONIZED_BIT) = 0x7b1a04c801c0 |
| 1873075 glBindBuffer(target = GL_COPY_WRITE_BUFFER, buffer = 30) |
| 1873076 glMapBufferRange(target = GL_COPY_WRITE_BUFFER, offset = 720, length = 72, access = GL_MAP_WRITE_BIT | GL_MAP_FLUSH_EXPLICIT_BIT | GL_MAP_UNSYNCHRONIZED_BIT) = 0x7b1a074302d0 |
| 1873077 glBindBuffer(target = GL_COPY_WRITE_BUFFER, buffer = 29) |
| 1873079 glFlushMappedBufferRange(target = GL_COPY_WRITE_BUFFER, offset = 0, length = 576) |
| 1873080 glUnmapBuffer(target = GL_COPY_WRITE_BUFFER) = GL_TRUE |
| 1873081 glBindBuffer(target = GL_COPY_WRITE_BUFFER, buffer = 30) |
| 1873083 glFlushMappedBufferRange(target = GL_COPY_WRITE_BUFFER, offset = 0, length = 72) |
| 1873084 glUnmapBuffer(target = GL_COPY_WRITE_BUFFER) = GL_TRUE |
| 1873085 glBindBuffer(target = GL_ARRAY_BUFFER, buffer = 29) |
| 1873096 glBindBuffer(target = GL_ELEMENT_ARRAY_BUFFER, buffer = 30) |
| 1873097 glDrawElementsBaseVertex(mode = GL_TRIANGLES, count = 36, type = GL_UNSIGNED_SHORT, indices = 0x2d0, basevertex = 240) |
| |
| In this app, buffer 29/30 get used like this starting from offset 0 every other |
| frame. The ``GL_ARB_sync`` fence is used to make sure that the GPU has reached the |
| start of the previous frame before we go unsynchronized writing over the n-2 |
| frame's buffer. |
| |
| Borderlands 2 |
| ============= |
| |
| .. code-block:: console |
| |
| 3561998 glFlush() |
| 3562004 glXSwapBuffers(dpy = 0xbaf0f90, drawable = 23068705) |
| 3562006 glClientWaitSync(sync = 0x231c2ab0, flags = GL_SYNC_FLUSH_COMMANDS_BIT, timeout = 10000000000) = GL_ALREADY_SIGNALED |
| 3562007 glDeleteSync(sync = 0x231c2ab0) |
| 3562008 glFenceSync(condition = GL_SYNC_GPU_COMMANDS_COMPLETE, flags = 0) = 0x231aadc0 |
| |
| 3562050 glBindBufferARB(target = GL_ARRAY_BUFFER, buffer = 1193) |
| 3562051 glMapBufferRange(target = GL_ARRAY_BUFFER, offset = 0, length = 1792, access = GL_MAP_WRITE_BIT | GL_MAP_INVALIDATE_BUFFER_BIT) = 0xde056000 |
| 3562053 glUnmapBufferARB(target = GL_ARRAY_BUFFER) = GL_TRUE |
| 3562054 glBindBufferARB(target = GL_ARRAY_BUFFER, buffer = 1194) |
| 3562055 glMapBufferRange(target = GL_ARRAY_BUFFER, offset = 0, length = 1280, access = GL_MAP_WRITE_BIT | GL_MAP_INVALIDATE_BUFFER_BIT) = 0xd9426000 |
| 3562057 glUnmapBufferARB(target = GL_ARRAY_BUFFER) = GL_TRUE |
| [... unrelated draws] |
| 3563051 glBindBufferARB(target = GL_ARRAY_BUFFER, buffer = 1193) |
| 3563064 glBindBufferARB(target = GL_ELEMENT_ARRAY_BUFFER, buffer = 875) |
| 3563065 glDrawElementsInstancedARB(mode = GL_TRIANGLES, count = 72, type = GL_UNSIGNED_SHORT, indices = NULL, instancecount = 28) |
| |
| The ``GL_ARB_sync`` fence ensures that the GPU has started frame n-1 before the CPU |
| starts on the current frame. |
| |
| This sequence of buffer uploads appears in each frame with the same buffer |
| names, so you do need to handle the ``GL_MAP_INVALIDATE_BUFFER_BIT`` as a |
| reallocate if the buffer is GPU-busy (it wasn't in this trace capture) to avoid |
| stalls on the n-1 frame completing. |
| |
| Note that this is just one small buffer. Most of the vertex data goes through a |
| ``glBufferSubData()``/``glDraw*()`` path with the VBO used across multiple |
| frames, with a ``glBufferData()`` when needing to wrap. |
| |
| Buffer mapping conclusions |
| -------------------------- |
| |
| * Non-blitting drivers must track the valid range of a freshly allocated buffer |
| as it gets uploaded in ``pipe_transfer_map()`` and avoid stalling on the GPU |
| when mapping an undefined portion of the buffer when ``glBufferSubData()`` is |
| interleaved with drawing. |
| |
| * Non-blitting drivers must reallocate storage on ``glBufferData(NULL)`` so that |
| the following ``glBufferSubData()`` won't stall. That ``glBufferData(NULL)`` |
| call will appear in the driver as an ``invalidate_resource()`` call if |
| ``PIPE_CAP_INVALIDATE_BUFFER`` is available. (If that flag is not set, then |
| mesa/st will create a new pipe_resource for you). Storage reallocation may be |
| skipped if you for some reason know that the buffer is idle, in which case you |
| can just empty the valid region. |
| |
| * Blitting drivers must use the ``transfer_flush_region()`` region |
| instead of the mapped range when ``PIPE_MAP_FLUSH_EXPLICIT`` is set, to avoid |
| blitting too much data. (When that bit is unset, you just blit the whole |
| mapped range at unmap time.) |
| |
| * Buffer valid range tracking in non-blitting drivers must use the |
| ``transfer_flush_region()`` region instead of the mapped range when |
| ``PIPE_MAP_FLUSH_EXPLICIT`` is set, to avoid excess stalls. |
| |
| * Buffer valid range tracking doesn't need to be fancy, "number of bytes |
| valid starting from 0" is sufficient for all examples found. |
| |
| * Use the ``util_debug_callback`` to report stalls on buffer mapping to ease |
| debug. |
| |
| * Buffer binding points are not useful for tuning buffer placement (See all the |
| ``PIPE_COPY_WRITE_BUFFER`` instances), you have to track the actual usage |
| history of a GL BO name. mesa/st does this for optimizing its state updates |
| on reallocation in the ``!PIPE_CAP_INVALIDATE_BUFFER`` case, and if you set |
| ``PIPE_CAP_INVALIDATE_BUFFER`` then you have to flag your own internal state |
| updates (VBO addresses, XFB addresses, texture buffer addresses, etc.) on |
| reallocation based on usage history. |
