[LLVMdev] [RFC] Less memory and greater maintainability for debug info IR
Sean Silva
chisophugis at gmail.com
Wed Oct 15 14:32:48 PDT 2014
On Wed, Oct 15, 2014 at 2:31 PM, Eric Christopher <echristo at gmail.com>
wrote:
> On Wed, Oct 15, 2014 at 2:30 PM, Sean Silva <chisophugis at gmail.com> wrote:
> >
> >
> > 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.
> >
>
> This is likely what we've already discussed and was handled a long
> while ago now.
>
>
I was reading the thread in sequential order (and replying without
finishing). derp.
-- Sean Silva
> -eric
>
> > -- 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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