[LLVMdev] [RFC] Less memory and greater maintainability for debug info IR
Sean Silva
chisophugis at gmail.com
Wed Oct 15 14:30:11 PDT 2014
On Mon, Oct 13, 2014 at 7:01 PM, Eric Christopher <echristo at gmail.com>
wrote:
> On Mon, Oct 13, 2014 at 6:59 PM, Sean Silva <chisophugis at gmail.com> wrote:
> > For those interested, I've attached some pie charts based on Duncan's
> data
> > in one of the other posts; successive slides break down the usage
> > increasingly finely. To my understanding, they represent the number of
> > Value's (and subclasses) allocated.
> >
> > On Mon, Oct 13, 2014 at 3:02 PM, Duncan P. N. Exon Smith
> > <dexonsmith at apple.com> wrote:
> >>
> >> In r219010, I merged integer and string fields into a single header
> >> field. By reducing the number of metadata operands used in debug info,
> >> this saved 2.2GB on an `llvm-lto` bootstrap. I've done some profiling
> >> of DW_TAGs to see what parts of PR17891 and PR17892 to tackle next, and
> >> I've concluded that they will be insufficient.
> >>
> >> Instead, I'd like to implement a more aggressive plan, which as a
> >> side-effect cleans up the much "loved" debug info IR assembly syntax.
> >>
> >> At a high-level, the idea is to create distinct subclasses of `Value`
> >> for each debug info concept, starting with line table entries and moving
> >> on to the DIDescriptor hierarchy. By leveraging the use-list
> >> infrastructure for metadata operands -- i.e., only using value handles
> >> for non-metadata operands -- we'll improve memory usage and increase
> >> RAUW speed.
> >>
> >> My rough plan follows. I quote some numbers for memory savings below
> >> based on an -flto -g bootstrap of `llvm-lto` (i.e., running `llvm-lto`
> >> on `llvm-lto.lto.bc`, an already-linked bitcode file dumped by ld64's
> >> -save-temps option) that currently peaks at 15.3GB.
> >
> >
> > Stupid question, but when I was working on LTO last Summer the primary
> > culprit for excessive memory use was due to us not being smart when
> linking
> > the IR together (Espindola would know more details). Do we still have
> that
> > problem? For starters, how does the memory usage of just llvm-link
> compare
> > to the memory usage of the actual LTO run? If the issue I was seeing last
> > Summer is still there, you should see that the invocation of llvm-link is
> > actually the most memory-intensive part of the LTO step, by far.
> >
>
> This is vague. Could you be more specific on where you saw all of the
> memory?
>
Running `llvm-link *.bc` would OOM a machine with 64GB of RAM (with -g;
without -g it completed with much less). The increasing could be easily
watched on the system "process monitor" in real time.
-- Sean Silva
>
> -eric
>
> >
> > Also, you seem to really like saying "peak" here. Is there a definite
> peak?
> > When does it occur?
> >
> >
> >>
> >>
> >> 1. Introduce `MDUser`, which inherits from `User`, and whose `Use`s
> >> must all be metadata. The cost per operand is 1 pointer, vs. 4
> >> pointers in an `MDNode`.
> >>
> >> 2. Create `MDLineTable` as the first subclass of `MDUser`. Use normal
> >> fields (not `Value`s) for the line and column, and use `Use`
> >> operands for the metadata operands.
> >>
> >> On x86-64, this will save 104B / line table entry. Linking
> >> `llvm-lto` uses ~7M line-table entries, so this on its own saves
> >> ~700MB.
> >>
> >>
> >> Sketch of class definition:
> >>
> >> class MDLineTable : public MDUser {
> >> unsigned Line;
> >> unsigned Column;
> >> public:
> >> static MDLineTable *get(unsigned Line, unsigned Column,
> >> MDNode *Scope);
> >> static MDLineTable *getInlined(MDLineTable *Base, MDNode
> >> *Scope);
> >> static MDLineTable *getBase(MDLineTable *Inlined);
> >>
> >> unsigned getLine() const { return Line; }
> >> unsigned getColumn() const { return Column; }
> >> bool isInlined() const { return getNumOperands() == 2; }
> >> MDNode *getScope() const { return getOperand(0); }
> >> MDNode *getInlinedAt() const { return getOperand(1); }
> >> };
> >>
> >> Proposed assembly syntax:
> >>
> >> ; Not inlined.
> >> !7 = metadata !MDLineTable(line: 45, column: 7, scope: metadata
> >> !9)
> >>
> >> ; Inlined.
> >> !7 = metadata !MDLineTable(line: 45, column: 7, scope: metadata
> >> !9,
> >> inlinedAt: metadata !10)
> >>
> >> ; Column defaulted to 0.
> >> !7 = metadata !MDLineTable(line: 45, scope: metadata !9)
> >>
> >> (What colour should that bike shed be?)
> >>
> >> 3. (Optional) Rewrite `DebugLoc` lookup tables. My profiling shows
> >> that we have 3.5M entries in the `DebugLoc` side-vectors for 7M line
> >> table entries. The cost of these is ~180B each, for another
> >> ~600MB.
> >>
> >> If we integrate a side-table of `MDLineTable`s into its uniquing,
> >> the overhead is only ~12B / line table entry, or ~80MB. This saves
> >> 520MB.
> >>
> >> This is somewhat perpendicular to redesigning the metadata format,
> >> but IMO it's worth doing as soon as it's possible.
> >>
> >> 4. Create `GenericDebugMDNode`, a transitional subclass of `MDUser`
> >> through an intermediate class `DebugMDNode` with an
> >> allocation-time-optional `CallbackVH` available for referencing
> >> non-metadata. Change `DIDescriptor` to wrap a `DebugMDNode` instead
> >> of an `MDNode`.
> >>
> >> This saves another ~960MB, for a running total of ~2GB.
> >
> >
> > 2GB (out of 15.3GB i.e. ~13%) seems pretty pathetic savings when we have
> a
> > single pie slice near 40% of the # of Value's allocated and another at
> 21%.
> > Especially this being "step 4".
> >
> > As a rough back of the envelope calculation, dividing 15.3GB by ~24
> million
> > Values gives about 600 bytes per Value. That seems sort of excessive
> (but is
> > it realistic?). All of the data types that you are proposing to shrink
> fall
> > far short of this "average size", meaning that if you are trying to
> reduce
> > memory usage, you might be looking in the wrong place. Something smells
> > fishy. At the very least, this would indicate that the real memory usage
> is
> > elsewhere.
> >
> > A pie chart breaking down the total memory usage seems essential to have
> > here.
> >
> >>
> >>
> >> Proposed assembly syntax:
> >>
> >> !7 = metadata !GenericDebugMDNode(tag: DW_TAG_compile_unit,
> >> fields: "0\00clang 3.6\00...",
> >> operands: { metadata !8, ...
> })
> >>
> >> !7 = metadata !GenericDebugMDNode(tag: DW_TAG_variable,
> >> fields: "global_var\00...",
> >> operands: { metadata !8, ...
> },
> >> handle: i32* @global_var)
> >>
> >> This syntax pulls the tag out of the current header-string, calls
> >> the rest of the header "fields", and includes the metadata operands
> >> in "operands".
> >>
> >> 5. Incrementally create subclasses of `DebugMDNode`, such as
> >> `MDCompileUnit` and `MDSubprogram`. Sub-classed nodes replace the
> >> "fields" and "operands" catch-alls with explicit names for each
> >> operand.
> >>
> >> Proposed assembly syntax:
> >>
> >> !7 = metadata !MDSubprogram(line: 45, name: "foo", displayName:
> >> "foo",
> >> linkageName: "_Z3foov", file:
> metadata
> >> !8,
> >> function: i32 (i32)* @foo)
> >>
> >> 6. Remove the dead code for `GenericDebugMDNode`.
> >>
> >> 7. (Optional) Refactor `DebugMDNode` sub-classes to minimize RAUW
> >> traffic during bitcode serialization. Now that metadata types are
> >> known, we can write debug info out in an order that makes it cheap
> >> to read back in.
> >>
> >> Note that using `MDUser` will make RAUW much cheaper, since we're
> >> using the use-list infrastructure for most of them. If RAUW isn't
> >> showing up in a profile, I may skip this.
> >>
> >> Does this direction seem reasonable? Any major problems I've missed?
> >
> >
> > You need more data. Right now you have essentially one data point, and
> it's
> > not even clear what you measured really. If your goal is saving memory, I
> > would expect at least a pie chart that breaks down LLVM's memory usage
> (not
> > just # of allocations of different sorts; an approximation is fine, as
> long
> > as you explain how you arrived at it and in what sense it approximates
> the
> > true number).
> >
> > Do the numbers change significantly for different projects? (e.g.
> Chromium
> > or Firefox or a kernel or a large app you have handy to compile with
> LTO?).
> > If you have specific data you want (and a suggestion for how to gather
> it),
> > I can also get your numbers for one of our internal games as well.
> >
> > Once you have some more data, then as a first step, I would like to see
> an
> > analysis of how much we can "ideally" expect to gain (back of the
> envelope
> > calculations == win).
> >
> > -- Sean Silva
> >
> >>
> >>
> >> _______________________________________________
> >> LLVM Developers mailing list
> >> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >
> >
>
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