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// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.
//
// --------------------------------------------------------------------------------------------------
// PerfDefaults.h
//
//
// Implementation of the "PerformanceScenario" config option, which defines a workload-specific
// set of performance defaults.
//
// The motivation is that every release we work closely with a partner to understand the performance
// of their scenario and provide CLR behavior to improve it. Sometimes we are able to give them
// an entirely different build of the CLR (because it happens to be on a different architecture, or
// it needs to have a small size, etc), but not always. Sometimes we are able to change the
// runtime to have the new behavior by default, but not always. When neither applies, we fall to
// the ugly option of a set of one-off config values for the scenario (or machine administrators)
// to opt in. This then creates the problem of a huge number of generic knobs that any application
// can use in any combination. It also means that as we add additional improvements for the same
// scenario, we need to ask developers or machine administrators to keep up with the growing list.
//
// Our solution is to opt-in to a set of performance settings all at once and based on the workload.
// When a recognized 'PerformanceScenario' is specified in a config file, we want the runtime to
// operate with a defined set of performance settings. This set may evolve over time to
// automatically provide additional behavior to the same workload, just as we would for runtime
// defaults. Note however that a key design point is that in any conflict between these performance
// settings and one that is explicitly set by the user, the explicit user setting wins. Hence the
// name PerformanceDefaults -- it is as if we had created a separate build of the runtime for a
// particular scenario, changing only the default performance behavior, and continued to let people
// opt in or opt-out individually to those settings that are important and general-purpose enough
// to have an individual setting.
//
// To add a scenario,
// Update the PerformanceScenario enum and InitializeForScenario so that it is recognized
//
// To add an overridden host STARTUP_FLAG,
// Update the switch statement in GetModifiedStartupFlags; see note about IsConfigOptionSpecified
//
// To add an overridden CLRConfig default value,
// Mark it with LookupOption CLRConfig::MayHavePerformanceDefault in clrconfigvalues.h
// Update the switch statement in LookupConfigValue
// If a new CLRConfig option, decide if it needs to be a general-purpose switch (hopefully no)
// and if not create it as INTERNAL_
//
// --------------------------------------------------------------------------------------------------
#ifndef __PerfDefaults_h__
#define __PerfDefaults_h__
class PerformanceDefaults
{
public:
static void InitializeForScenario(__in_opt LPWSTR scenarioName);
// Called at runtime startup to allow host-specified STARTUP_FLAGS to be overridden when running
// under a scenario.
//
// Note that this does not follow the model of "only override if not specified." By the nature
// of the startup flags being bits, they are all specified (as on or off). We have no way of
// knowing which the host actually care about and which were left as the runtime default
// out of lack of concern. However we found that so far the specified startup flags are hard
// coded by the host (no way for the user to override at run time) and so we can build in
// to the set of defaults which the host allows us to override.
//
// In the future, if a host needs us to be able to override startup flags conditionally, the
// solution would be a new hosting API ICLRRuntimeInfo::SetOverridableStartupFlags.
static STARTUP_FLAGS GetModifiedStartupFlags(STARTUP_FLAGS originalFlags);
// Called by the CLRConfig system whenever a 'MayHavePerformanceDefault' config option is about
// to resolve to its runtime default (i.e. none of the corresponding environment variable,
// registry, or config file values were specified).
static BOOL LookupConfigValue(LPCWSTR name, DWORD *pValue);
private:
enum PerformanceScenario
{
Uninitialized,
None,
HighDensityWebHosting,
};
static PerformanceScenario s_Scenario;
};
#endif //__PerfDefaults_h__
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