[Bug 101592] Very slow performance when rendering scenes with transparency, probably caused by excessive copying (intel_miptree_map())

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=101592

            Bug ID: 101592
           Summary: Very slow performance when rendering scenes with
                    transparency, probably caused by excessive copying
                    (intel_miptree_map())
           Product: Mesa
           Version: 17.1
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Drivers/DRI/i965
          Assignee: intel-3d-bugs at lists.freedesktop.org
          Reporter: sdh4 at iastate.edu
        QA Contact: intel-3d-bugs at lists.freedesktop.org

I am seeing very poor rendering performance when drawing scenes with
transparency. 

This is with Mesa 17.1.3 as distributed with Fedora 26 beta
(mesa-dri-drivers-17.1.3-2.fc26). Testing on Intel HD Graphics 620, lspci shows
it as "Device 5916"

Rendering has slowed by 10-100x and is significantly (10x) slower than pure
software rendering (via LIBGL_ALWAYS_SOFTWARE)

Specifically, I see very high CPU usage during rendering in the
intel_miptree_map(), intel_offset_S8(), and intel_miptree_unmap() routines.
Via "perf record" and "perf report", the locus of the CPU usage is clear:

  91.34%  dg_scope_sa  i965_dri.so               [.] intel_miptree_map
   4.53%  dg_scope_sa  i965_dri.so               [.] intel_offset_S8
   1.25%  dg_scope_sa  i965_dri.so               [.] intel_miptree_unmap

>From typical stack backtraces (such as pasted below) it
seems to be spending most of its time mapping (with
intel_mipmap_tree.c:intel_miptree_map_s8()) or unmapping a rather large
(width=815, height=534) miptree (which translates to the size of the entire
FreeGLUT window being rendered into)

Notice that all of this is inside a call to glutBitmapCharacter()... i.e. 
it appears that every character being drawn causes creation and destruction of
a full-window miptree. 

... Hence the terrible performance. 

Stack trace follows

#0  0x00007f1dfabe4ae1 in intel_miptree_unmap_s8 (slice=<optimized out>, 
    level=0, map=0x12028d0, mt=0x1202dc0, brw=0x7f1e02fd1040)
    at intel_mipmap_tree.c:2728
#1  intel_miptree_unmap (brw=brw at entry=0x7f1e02fd1040, mt=0x1202dc0, level=0, 
    slice=<optimized out>) at intel_mipmap_tree.c:3101
#2  0x00007f1dfabe23b7 in intel_unmap_renderbuffer (ctx=0x7f1e02fd1040, 
    rb=0x12023d0) at intel_fbo.c:217
#3  0x00007f1dfa9758de in unmap_attachment (ctx=ctx at entry=0x7f1e02fd1040, 
    fb=fb at entry=0x1201d30, buffer=buffer at entry=BUFFER_STENCIL)
    at swrast/s_renderbuffer.c:609
#4  0x00007f1dfa9760f0 in _swrast_unmap_renderbuffers (ctx=0x7f1e02fd1040)
    at swrast/s_renderbuffer.c:689
#5  0x00007f1dfa965318 in swrast_render_finish (ctx=0x7f1e02fd1040)
    at swrast/s_context.h:379
#6  _swrast_Bitmap (ctx=ctx at entry=0x7f1e02fd1040, px=426, py=py at entry=263, 
    width=width at entry=9, height=height at entry=16, unpack=0x7f1e02fecf30, 
    bitmap=0x7f1e012e4ac1 "") at swrast/s_bitmap.c:135
#7  0x00007f1dfa9a686d in _mesa_meta_Bitmap (ctx=ctx at entry=0x7f1e02fd1040, 
    x=x at entry=426, y=y at entry=263, width=width at entry=9, height=height at entry=16, 
    unpack=unpack at entry=0x7f1e02fecf30, bitmap1=0x7f1e012e4ac1 "")
    at drivers/common/meta.c:2355
#8  0x00007f1dfabe7fac in intelBitmap (ctx=<optimized out>, x=426, y=263, 
    width=<optimized out>, height=16, unpack=0x7f1e02fecf30, 
---Type <return> to continue, or q <return> to quit--- 
    pixels=0x7f1e012e4ac1 "") at intel_pixel_bitmap.c:358
#9  0x00007f1dfa820b5f in _mesa_Bitmap (width=9, height=16, 
    xorig=<optimized out>, yorig=<optimized out>, xmove=<optimized out>, 
    ymove=<optimized out>, bitmap=0x7f1e012e4ac1 "") at main/drawpix.c:347
#10 0x00007f1e012d149c in glutBitmapCharacter () from /lib64/libglut.so.3
#11 0x00000000004111df in DrawString (String=0x1202620 "GOWJOT")
    at scope_draw.c:79
#12 0x0000000000418260 in DrawWaveformImage (c=0xecbf40, pixelsperdiv=66, 
    usex=2, usey=3, usewidth=792, useheight=528, centerline=267)
    at scope_drawwfm.c:985
#13 0x0000000000418f5e in DrawWaveform (c=0xecbf40, pixelsperdiv=66, usex=2, 
    usey=3, usewidth=792, useheight=528, centerline=267, isSelected=1)
    at scope_drawwfm.c:1161
#14 0x000000000040df0f in ScopeDisplayFunc () at scope_callbacks.c:1361
#15 0x00007f1e012cd3c1 in fghRedrawWindow () from /lib64/libglut.so.3
#16 0x00007f1e012cd84c in fghcbProcessWork () from /lib64/libglut.so.3
#17 0x00007f1e012ceaa9 in fgEnumSubWindows () from /lib64/libglut.so.3
#18 0x00007f1e012ce9b9 in fgEnumWindows () from /lib64/libglut.so.3
#19 0x00007f1e012cd931 in glutMainLoopEvent () from /lib64/libglut.so.3
#20 0x00007f1e012cd9d4 in glutMainLoop () from /lib64/libglut.so.3
#21 0x00000000004085f8 in main (argc=2, argv=0x7ffcf2509128) at scope.c:633

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